Here are the main reasons your headset is not installing apps with Advanced Installer or apps aren't showing up:

• USB cable. Try a different one, preferably brand new. <-THIS IS THE NUMBER ONE REASON!
• USB port. Try a different port.
• Debug mode is bugged or needs re-verification in the Meta Quest app. Toggle it off and back on again to be sure.
• Delete any secondary account on the headset, as that account can't sideload.
• Restarting the headset may help, too.
• The device you're using has driver issues, so restart the PC.
• It is worth considering a factory reset of the Quest if you've tried everything else.

In-VR app.

• Try an Android phone or a different PC with the web installer to install the app.
• Check in the Unknown sources from the 'All' drop-down in the app library for sideloaded apps.

Be sure to confirm the USB debugging prompt in the headset before you can install the app or connect via USB to Advanced Installer. If you don't see this prompt, one of the issues above is the culprit, and you won't be able to continue, Try A - B testing with everything you can associated to the point above to resolve your issue.

If you need more ADB or debugging help, Meta Support can also help, as it's used in their ODH (Meta/Oculus developer hub). You could also install that and see the same issue with connecting.

If you're unable to toggle debug mode again, contact Meta support to look into your account. Be sure you've added a phone number and 2FA to your Meta account or you won't be able to toggle debug mode on.

All the best!