"The error message you're receiving indicates a problem with the SSL certificate for the ProtonVPN repository. More specifically, the error "SSL certificate problem: self-signed certificate in certificate chain" suggests that there's a self-signed certificate somewhere in the certificate chain, which is generally considered insecure and can't be verified.

Here are a few possible solutions:

1. **Verify Your System Time**: An incorrect system time can cause SSL errors. Check to make sure your system time is accurate.

2. **Try Disabling SSL Verification Temporarily**: This isn't recommended for long-term solutions because it can leave your system vulnerable. But, for a quick fix or to see if SSL verification is the issue, you can try this. You'll need to modify your DNF configuration file to disable SSL verification. Here's how:

Open the DNF configuration file in a text editor with root permissions. For example, you can use `sudo` with `nano` like this:
```bash
sudo nano /etc/dnf/dnf.conf
```
Then, add or modify the `sslverify` line to be `False`:
```bash
sslverify=False
```
Save and exit the file. Then try running `sudo dnf update` again.

Remember to set `sslverify` back to `True` once you're done to keep your system secure.

3. **Contact ProtonVPN Support**: If none of the above solutions work, there might be an issue with the repository or the certificate that ProtonVPN needs to fix. Reach out to their support for help.

Remember, solution 2 is not recommended for long-term use as it disables SSL verification, which can leave your system vulnerable. It's primarily a troubleshooting step to help determine where the issue is coming from."