Android 11
Android 11 changed some things about how apps can access storage, which breaks the way emulators and frontends used to access roms and talk to each other. This doesn't affect every emulator, it depends on how the emulators work and when they were last updated, but a few emulators like M64Plus FZ and PPSSPP no longer support launching games from frontends using the old method.

Note that it's also possible for this issue to appear in earlier versions of Android for a recently updated emulator, but it's more likely to happen in Android 11.
SAF Proxy
To get around this storage access issue and launch games from ARC Browser in the latest versions of these emulators on Android 11 and later you need a small helper app.
  • Download SAF Proxy 1.1 here and install it on your device.
  • Open SAF Proxy and grant it access to your rom directory/directories.
  • In ARC Browser, use the template for the emulator that has "(via SAF Proxy)" in the name.
  • Make sure you're using the latest version of the emulator.
(This functionality will likely be integrated in ARC Browser in the future, but right now you need to use this app)