Why can’t I move photos from a shared album to an unshared album in Photos app?

Apple isn't here (except for a few moderators), but maybe I or another user can help.

You did get yourself into a complicated situation by using a Shared Album to collect photos for both sharing and non-shared purposes. I'll try to explain.

The normal Album system doesn't work by moving or copying but by adding photos to Albums by reference. You can drag one photo to ten Albums, but it is only one photo in the library referred to by ten Albums.

Unfortunately, Shared Albums aren't really part of this system. They exist outside the Library proper. Photos get there by a process that is part copying and part alteration, reducing large photos to smaller pixel counts and making similar changes to videos. Getting photos back from the Shared Album system requires Importing them into the Library proper before you can add them to normal Albums.

In any event, rather than trying to get all of your selected photos back into a normal Album and into Facebook from there, may I suggest a different route?

• Select all of the photos in the Shared Album.
• Export all of them to a temporary Finder folder, perhaps in your Desktop folder.
• Upload to Facebook from that temporary folder.
• Delete the temporary folder when you are done.

You may need to play with export options to get file names that sort well, depending on how you want the pictures ordered in the temporary folder.

My condolences on the passing away of your mother. It is hard to have to deal with the inconsistencies in Photos when in mourning.

In addition to Mark's great explanation, there is one more thing you should consider before using Shared albums to store your photos: you are losing quality and metadata. Shared albums are using separate copies of your photos in your library, because the photos in the Shared album are optimized for sharing. Photos is downsizing them to a small file sized removing metadata, so we do not give away too much information when sharing the photos. So Photos needs to create a copy to preserve the original a high quality in our library. Photos in Shared albums are not duplicates of your photos, just special working copies for sharing.

If you want to use the photos on Facebook in a high quality, you need to find the photos again in your library, that you added to the shared album.