This error is certainly related with your device app permission.
The file storage permission is enabled, just like I meant before. I used another device just of recent, there was a request to allow Electrum to allow camera and media, I clicked on 'allow', but the same error occured after I allowed it. Which means it is not by default like I meant before, but I checked the media permission, and it was set on allowed, I allowed it before. When I first saw the error, I checked it and it was set on 'allow'.
For reference, the error will be different if you didn't grant it media storage permission, so the issue must be something else.
The error would be "
Cannot save backup without STORAGE permission".
Third question
How can it be possible to successfully use file backup on mobile Electrum to recover your wallet on mobile phone? I remember I wanted to try it in the past, but I was unable to.
The backup file is a complete copy of your wallet file. It should be compatible with PC Electrum and protected with the password that you've set.
The issue is, there's no option to load it to mobile Electrum.