Photo library from my external hard drive won't open with current macOS

This information may help. Be advised, it was updated in 2016. https://support.apple.com/en-us/HT202358

The error message you have shown can also appear, if you are using an outdated Photos icon in the Dock or an alias, still linked to an older version of Photos.

Try to launch the Photos.app installed in your Applications folder directly from Applications instead of using an alias or the Dock icon.

• Open the folder "Applications", then hold down the options key ⌥ while double clicking the Photos icon in Applications. Keep holding down the options key ⌥ firmly, until the library chooser dialog appears.
• Release the options key ⌥, then look at the list of Photos Library. Is the library shown, that you want to open? By default you Photos Libraries are in the pIctures folder, unless you create them somewhere else. Did you backup the Pictures folder three years ago?

I think you have put the application photos (from an older version of MacOS) onto the external drive, and not your photos library.

The application photos is like the application pages, or the application Microsoft word, where the library is the equivalent of a pages or word document. The library is the object that contains your photos. The application Photos is the app you use to open and view the contents of the library.

You need to find the actual library from your older MacOS version. This will most likely be called Photos Library.photoslibrary or similar. Whatever it is called, it will definately have a .photoslibrary extension.

Looks like you are trying to open the actual application rather than the library.

Looks for the PhotosLibrary.photoslibrary file, rather than the Photos.app icon.