Can't open Garageband files after upgrading to Monterey

The screenshot looks like a very old GarageBand 9 or 11 iLife version. You are getting the prompt to save the project, because GarageBand needs to upgrade the format.

the older projects may contain audio files in an unsupported legacy format, and many sounds may be missing, because GarageBand 10 has a different set of instruments. I have always been careful to keep the older GarageBand versions installed when upgrading to a new system, so the old instruments and loops have been saved and could be used with the newer GarageBand versions as well.

As you already looked at the package content, you may check the audio fils inside, if the current QuickTime player can play them at all.

Hi Riley Brown,

Thank you for participating in the Apple Support Communities. We understand you're having trouble opening older GarageBand files after upgrading macOS, and we'd like to assist. Does it allow you to save it with a slightly different name in a different folder location? If so, does your project file open after doing so? Also, this article has some steps that may help:

If GarageBand isn't working on your Mac

If these steps don't help, we recommend calling Apple Support for assistance.

Contact Apple for support and service

Take care.