Client Request library rebuild keeps repeating despite repair option

Do you have only one Library? Can you create a new Library? If so, does that Library read, with File>Import, the other Library?

Do you use iCloud Photos?

Have you tried these things?

• Restart the computer (of course)
• Re-start in Safe Mode. This bypasses certain potentially disruptive processes, and it often helps. Safe Mode is different for different computers, so see this: 

           Start up your Mac in safe mode - Apple Support 

• Rebuild your Photos Library--close Photos and re-launch by option-command-clicking the app icon. Rebuilding may cause a very long re-syncing if you are using iCloud Photos.
• Make a new user and see if the same problem recurs with it.  This is a bit harder, because a new user can't access a different user's stuff. You would need to move your Library out of your own user's folder up to the general "Users" folder. 

The idea is to find out if the problem is with the Library, with your account, or with caches and login items, and stuff like that. One of these steps may fix the problem, but their failure to work also helps figure out what's happening.

Let us know what happens…

The first thing you need to do is get an external drive that's about 2 - 2 ½ the size of your working drive and start a Time Machine backup program.

You can ignore the above if you don't value your photos and other user files.

If all else fails you could try using the paid version of PowerPhotos to merge your problem library into a new, empty Photos library.