Photos app won't open a library--- says that photos library is from a newer version of Photos...running Catalina 10.15.7

You will be seeing this error message for two possible reasons:

• If your Photos Libraries have been opened by a newer version of Photos, for example on a Mac running macOS 11 Big Sur. Did you perhaps downgrade the system back from Big Sur to Catalina?
• Or, if you are trying to launch Photos by clicking an older alias, for example in the Dock or an alias on the Desktop.

To be sure, you are running the latest version of Photos, installed by the system, open your Applications folder and select Photos there. Then hold down the options key ⌥ while double clicking Photos. Release the key when the library chooser dialog appears. You will see all compatible libraries found by Photos. Select the library you want to open.

That explains it then.

If it is not possible to upgrade to Big Sur because of the internal policy work, consider to install Big Sur on an external volume or in a separate partition.

This would allow you to upload the photos library to iCloud Photos, and from iCloud you can download it to a Mac with Catalina and downgrade the library back to a Catalina version.

I was storing all my photographs on an external HD using the photos App. That Photos icon now has a circle and a slash which means it cannot be selected using the manner described. How do I recover my photos on this HD? Even using my older computer with Catalina I am unable to access these photos.