Mac mini 2024 15.5 (24F74) how to get photo album to screen saver

Is the library you're looking for designated the System Photo Library?

Since "Use as System Photo Library" is not grayed out (meaning it could be changed to the System Library,) this is not the current System Library. The System Library is the one that can communicate with other apps like Safari or Screen Saver, and it's the only one that can work with iCloud.

Apparently you have more than one Photos Library! You can see what Libraries Photos knows about by

closing Photos and then option-clicking on the Photos icon to bring up the Library Chooser:

This lists all the Libraries and tells you which is the System Library. It also shows the path to each Library. You can open them and see what each is like.

You might want to switch which Library is the System Library in the Setting>General dialog you showed. Or you may want to merge two Libraries. There are a couple of ways to do the Merging, so let us know and we can help. We need to know if the album and folder structure is something you need to preserve in the merge.

What happens when you try? Which step fails?

Can you not select the "Photo Library..." option in Screen Saver in System Settings?

Do you not get a list of Albums to scroll through?

Can you not find the Album you want?

Can you not select the Album you want?

Does the screensaver not the display the photos in the Album?

Your screenshot is showing, the top level albums in your library. Are the albums you want in subfolders?

I noticed, when I tested this with my iCloud Photos Library, that the dialog to select an album needs several minutes to show all albums. The albums in nested subfolders appeared a long time later than the top-level albums. Perhaps you are not waiting long enough, if your library is large?

Go to Photos > Settings > General and look at what path is shown in Library Location. Does it really point into your Applications folder? The default would be in your Photos folder.