This is the biggest problem with these containers at the moment, you don't get any feedback what permissions are missing, first you need to understand this is a permission problem with the container itself and then to understand what permissions are missing and then apply them correctly.
For flatpak there is an api you can use to change permissions but for snap what I can remember that is not something you, the user, can change, that is up to the maintainer to enable them.
Applications like an IDE uses a lot of different resources so I gave up on using that as a flatpak, luckily Jetbrains ships their IDEs as tar.gz binary package you can use instead.
Flatpak works best when the application is very self contained, like Spotify, it streams music from an internet service, it doesn't require any special permissions.
I used Bitwarden as a flatpak, it had limited file access with one granted directory (Downloads), I was going to download an attachment from the Bitwarden application, the file saving dialog started one directory up from Downloads, you had to pick and open the Downloads directory first before saving, however I managed to save my attachment in that starting location outside of Downloads, some void directory that I never found.