NAME FSTYPE FSVER LABEL UUID FSAVAIL FSUSE% MOUNTPOINT
sda
├─sda1 ext4 1.0 rootMX21 fa0d630d-3523-40a3-89cf-af8eda7c5bc5 191.2G 4% /
└─sda2 swap 1 swapMX f76bb067-85a0-4664-9b11-4304f02c8d81 [SWAP]
sr0
nvme0n1
├─nvme0n1p1 vfat FAT32 ESP 284A-8C34
├─nvme0n1p2
├─nvme0n1p3 ntfs OS 90588A045889E8F4
├─nvme0n1p4 ntfs 40CABA47CABA38CC
├─nvme0n1p5 ntfs CA8C19938C197AE1
└─nvme0n1p6 ntfs Image 009029DE9029DB44

Change it to

Installing for x86_64-efi platform.
grub-install: warning: EFI variables are not supported on this system..
Installation finished. No error reported.

Restarted, but the Grub Customizer doesn't show anything different (such as an additional Windows).

When you reboot, and press f12 do you have an option to boot mx/debian in efi mode, or an option to boot from a file?
Post your /boot/grub/grub.cfg
Post your /etc/default/grub
Post your /etc/grub.d/40_custom

No boot options like that.

Post your /boot/grub/grub.cfg
bash: /boot/grub/grub.cfg: Permission denied

Post your /etc/default/grub
bash: /etc/default/grub: Permission denied

(both also when I go su level)

Post your /etc/grub.d/40_custom
/etc/grub.d/40_custom
# This file provides an easy way to add custom menu entries. Simply type the
# menu entries you want to add after this comment. Be careful not to change
# the 'exec tail' line above.

Post the output of

5.10.0-9-amd64

It should give you that message exactly once. The first time you use 'sudo'. After the first time then it should only ask for your password.

If you are seeing that message more than once then likely you are not entering your password correctly and sudo has not actually been run properly. Only after successfully running for your user (the first time) does sudo set the flag to no longer show that message.

Mount the efi partition to /boot/efi
post the output of the “ls /boot/efi/EFI”

Acronis Boot dell Microsoft mx

Which sounds good, because that drive has a Acronis installation.

Restarted, but it doesn't show in the Grub Customizer.
Just wondering... there is a Grub message for a split second on boot...

A s a Newbie, I don't really know how the Grub Customizer is supposed to integrate a Windows installation. But now, I can add "Chainloader", name it Windows, and select a partition. It then shows on boot. However, whatever I choose, I get a "partition not found" message.

Post the output of
cat /boot/grub/grub.cfg

$ cat /boot/grub/grub.cfg
#
# DO NOT EDIT THIS FILE
#
# It is automatically generated by grub-mkconfig using templates
# from /etc/grub.d and settings from /etc/default/grub
#

### BEGIN /etc/grub.d/00_header ###
if [ -s $prefix/grubenv ]; then
set have_grubenv=true
load_env
fi
if [ "${next_entry}" ] ; then
set default="${next_entry}"
set next_entry=
save_env next_entry
set boot_once=true
else
set default="0"
fi

if [ x"${feature_menuentry_id}" = xy ]; then
menuentry_id_option="--id"
else
menuentry_id_option=""
fi

export menuentry_id_option

if [ "${prev_saved_entry}" ]; then
set saved_entry="${prev_saved_entry}"
save_env saved_entry
set prev_saved_entry=
save_env prev_saved_entry
set boot_once=true
fi

function savedefault {
if [ -z "${boot_once}" ]; then
saved_entry="${chosen}"
save_env saved_entry
fi
}
function load_video {
if [ x$feature_all_video_module = xy ]; then
insmod all_video
else
insmod efi_gop
insmod efi_uga
insmod ieee1275_fb
insmod vbe
insmod vga
insmod video_bochs
insmod video_cirrus
fi
}

if [ x$feature_default_font_path = xy ] ; then
font=unicode
else
insmod part_msdos
insmod ext2
set root='hd0,msdos1'
if [ x$feature_platform_search_hint = xy ]; then
search --no-floppy --fs-uuid --set=root --hint-bios=hd0,msdos1 --hint-efi=hd0,msdos1 --hint-baremetal=ahci0,msdos1 fa0d630d-3523-40a3-89cf-af8eda7c5bc5
else
search --no-floppy --fs-uuid --set=root fa0d630d-3523-40a3-89cf-af8eda7c5bc5
fi
font="/usr/share/grub/unicode.pf2"
fi

if loadfont $font ; then
set gfxmode=1920x1080x8
load_video
insmod gfxterm
set locale_dir=$prefix/locale
set lang=en_US
insmod gettext
fi
terminal_output gfxterm
insmod part_msdos
insmod ext2
set root='hd0,msdos1'
if [ x$feature_platform_search_hint = xy ]; then
search --no-floppy --fs-uuid --set=root --hint-bios=hd0,msdos1 --hint-efi=hd0,msdos1 --hint-baremetal=ahci0,msdos1 fa0d630d-3523-40a3-89cf-af8eda7c5bc5
else
search --no-floppy --fs-uuid --set=root fa0d630d-3523-40a3-89cf-af8eda7c5bc5
fi
insmod gfxmenu
insmod jpeg
insmod png
set theme=($root)/boot/grub/themes/mx_linux/theme.txt
export theme
if [ "${recordfail}" = 1 ] ; then
set timeout=30
else
if [ x$feature_timeout_style = xy ] ; then
set timeout_style=menu
set timeout=7
# Fallback normal timeout code in case the timeout_style feature is
# unavailable.
else
set timeout=7
fi
fi
### END /etc/grub.d/00_header ###

### BEGIN /etc/grub.d/05_debian_theme ###
set menu_color_normal=cyan/blue
set menu_color_highlight=white/blue
### END /etc/grub.d/05_debian_theme ###

### BEGIN /etc/grub.d/10_linux ###
function gfxmode {
set gfxpayload="${1}"
}
set linux_gfx_mode=
export linux_gfx_mode
menuentry 'MX 21 Wildflower' --class mx --class gnu-linux --class gnu --class os $menuentry_id_option 'gnulinux-simple-fa0d630d-3523-40a3-89cf-af8eda7c5bc5' {
load_video
insmod gzio
if [ x$grub_platform = xxen ]; then insmod xzio; insmod lzopio; fi
insmod part_msdos
insmod ext2
set root='hd0,msdos1'
if [ x$feature_platform_search_hint = xy ]; then
search --no-floppy --fs-uuid --set=root --hint-bios=hd0,msdos1 --hint-efi=hd0,msdos1 --hint-baremetal=ahci0,msdos1 fa0d630d-3523-40a3-89cf-af8eda7c5bc5
else
search --no-floppy --fs-uuid --set=root fa0d630d-3523-40a3-89cf-af8eda7c5bc5
fi
echo 'Loading Linux 5.10.0-9-amd64 ...'
linux /boot/vmlinuz-5.10.0-9-amd64 root=UUID=fa0d630d-3523-40a3-89cf-af8eda7c5bc5 ro quiet splash
echo 'Loading initial ramdisk ...'
initrd /boot/initrd.img-5.10.0-9-amd64
}
submenu 'Advanced options for MX 21 Wildflower' $menuentry_id_option 'gnulinux-advanced-fa0d630d-3523-40a3-89cf-af8eda7c5bc5' {
menuentry 'MX 21 Wildflower, with Linux 5.10.0-9-amd64' --class mx --class gnu-linux --class gnu --class os $menuentry_id_option 'gnulinux-5.10.0-9-amd64-advanced-fa0d630d-3523-40a3-89cf-af8eda7c5bc5' {
load_video
insmod gzio
if [ x$grub_platform = xxen ]; then insmod xzio; insmod lzopio; fi
insmod part_msdos
insmod ext2
set root='hd0,msdos1'
if [ x$feature_platform_search_hint = xy ]; then
search --no-floppy --fs-uuid --set=root --hint-bios=hd0,msdos1 --hint-efi=hd0,msdos1 --hint-baremetal=ahci0,msdos1 fa0d630d-3523-40a3-89cf-af8eda7c5bc5
else
search --no-floppy --fs-uuid --set=root fa0d630d-3523-40a3-89cf-af8eda7c5bc5
fi
echo 'Loading Linux 5.10.0-9-amd64 ...'
linux /boot/vmlinuz-5.10.0-9-amd64 root=UUID=fa0d630d-3523-40a3-89cf-af8eda7c5bc5 ro quiet splash
echo 'Loading initial ramdisk ...'
initrd /boot/initrd.img-5.10.0-9-amd64
}
menuentry 'MX 21 Wildflower, with Linux 5.10.0-9-amd64 (systemd)' --class mx --class gnu-linux --class gnu --class os $menuentry_id_option 'gnulinux-5.10.0-9-amd64-init-systemd-fa0d630d-3523-40a3-89cf-af8eda7c5bc5' {
load_video
insmod gzio
if [ x$grub_platform = xxen ]; then insmod xzio; insmod lzopio; fi
insmod part_msdos
insmod ext2
set root='hd0,msdos1'
if [ x$feature_platform_search_hint = xy ]; then
search --no-floppy --fs-uuid --set=root --hint-bios=hd0,msdos1 --hint-efi=hd0,msdos1 --hint-baremetal=ahci0,msdos1 fa0d630d-3523-40a3-89cf-af8eda7c5bc5
else
search --no-floppy --fs-uuid --set=root fa0d630d-3523-40a3-89cf-af8eda7c5bc5
fi
echo 'Loading Linux 5.10.0-9-amd64 ...'
linux /boot/vmlinuz-5.10.0-9-amd64 root=UUID=fa0d630d-3523-40a3-89cf-af8eda7c5bc5 ro quiet splash init=/lib/systemd/systemd
echo 'Loading initial ramdisk ...'
initrd /boot/initrd.img-5.10.0-9-amd64
}
}

### END /etc/grub.d/10_linux ###

### BEGIN /etc/grub.d/20_linux_xen ###

### END /etc/grub.d/20_linux_xen ###

### BEGIN /etc/grub.d/20_memtest86+ ###
if [ "${grub_platform}" == "pc" ]; then
true
menuentry "Memory test (memtest86+)" {
insmod part_msdos
insmod ext2
set root='hd0,msdos1'
if [ x$feature_platform_search_hint = xy ]; then
search --no-floppy --fs-uuid --set=root --hint-bios=hd0,msdos1 --hint-efi=hd0,msdos1 --hint-baremetal=ahci0,msdos1 fa0d630d-3523-40a3-89cf-af8eda7c5bc5
else
search --no-floppy --fs-uuid --set=root fa0d630d-3523-40a3-89cf-af8eda7c5bc5
fi
linux16 /boot/memtest86+.bin
}
fi
### END /etc/grub.d/20_memtest86+ ###

### BEGIN /etc/grub.d/30_os-prober ###
### END /etc/grub.d/30_os-prober ###

### BEGIN /etc/grub.d/30_uefi-firmware ###
### END /etc/grub.d/30_uefi-firmware ###

### BEGIN /etc/grub.d/40_custom ###
# This file provides an easy way to add custom menu entries. Simply type the
# menu entries you want to add after this comment. Be careful not to change
# the 'exec tail' line above.
### END /etc/grub.d/40_custom ###

### BEGIN /etc/grub.d/41_custom ###
if [ -f ${config_directory}/custom.cfg ]; then
source ${config_directory}/custom.cfg
elif [ -z "${config_directory}" -a -f $prefix/custom.cfg ]; then
source $prefix/custom.cfg;
fi
### END /etc/grub.d/41_custom ###