Aravisian post for the compatibility layer install is awesome, please do that.
But what FrenchPress also said is very true about the sandbox nature of FLATPAK's.
If your using any FLATPAK APPS, the easiest way to provide them permission to see mounted drives, is by using FLATSEAL
https://flathub.org/apps/details/com.github.tchx84.Flatseal
You can learn how to use it from Youtube video's. But essentially its a FLATPAK manager APP. One of its features, will allow you to provide permission to FLATPAKS.
StarTreker
Your Tech Support Guru