Thank you @HappyTux and @Bonzoo!
I used both of your advices and finally I can open the folder.
Using
ls -l ~/ and simultaneously checking permissions from inside file manager I learnt and confirmed that downloads folder was owned by
_apt and group was
root.
Then I changed the file and group ownerships by following the instructions I found in
https://linuxcommand.org/lc3_lts0090.php
And later I changed the permissions from inside the file manager. I tried the terminal method as explained in above article but failed.
But it's all done now. I can't thank enough both of you! Thanks again!