Few quick checks:

- have you rebooted after installing flatpak? If you didn't, please do so

- have you installed Angelfish using --user?

- what does `flatpak list` return in terminal?

- Try to enter Flatpak environment by running: flatpak run --command=sh org.kde.mobile.angelfish

The last command should give you a prompt inside Flatpak. If something is wrong, it will fail.