Photos app on Mac won't open

If your library is not an iCloud Photos Library you need to restore the most recent backup of your library that you made before the upgrade to Monterey. It is simply not possible to open a Photos Library from a newer system version in an older version of Photos, because the older version has not been updated to handle the new format of the newer libraries. That is why we need to make a backup copy of the Photos Library in the old format before upgrading to a new system version.

Hi - So your beta version upgraded the library to the beta format. Now your current version cannot open it.

Do you have your library also in iCloud? If so you're in luck. You can create a new empty library, and sync that with iCloud and it will download all your photos.

If you don't then the only way to downgrade the library - is to use iCloud. You will need a machine running the beta version of Monterey, on that machine sync your upgraded library to iCloud (you will need at least a month of a plan big enough for your complete library). Then back to your current MacOS - new empty library, and sync to iCloud again.

For more detail, see this user tip by Leonie

https://discussions.apple.com/docs/DOC-250000297