Photos - repairing the library

I take it, you already tried to restart your Mac?

If the library is on your internal drive, in your Pictures folder, try if repairing your user Home folder will allow the Library Repair to succeed. For some users running Mojave this helped, because Mojave is more restrictive with respect to user permissions and file ownership than the previous systems.

See: Resolve issues caused by changing the permissions of items in your home folder - Apple Support

You may also check the System Preferences > Security & Privacy and enable "Full Disk Access" for Photos.

Also, make sure that you are repairing the correct library. Sometimes Photos will switch between libraries, if it cannot access the correct library. After repairing the permissions try the Library Repair like this:

• Select your main Photos Library in the Finder.
• Hold down the key combination ⌥⌘ while double clicking the Library. Keep holding down the two keys firmly, until the library chooser dialog appears. This will ensure, that Photos will repair the selected library and not the last opened library.

Photos needs plenty of free storage for the repair. make sure, your Mac is not low on storage, when you repair the library.

This can happen, if the library is damaged beyond repair, or if the library is on an external drive, that is not compatible with Photos or is failing. Or if you imported media files that are incompatible with Photos and cannot be processed. or if there is not enough free storage on the drive with the library to complete the repair.

Can you tell us more about your Mac and your Photos Library?

• Why has the repair become necessary? Did Photos crash or did you restore a library from backup? Did you run any cleaning applications?
• Which system is installed on your Mac? Where is your Photos Library stored?
• If your library is on an external drive, what is the file system format? Are any backups on the same drive as your library?

Great! You're welcome 😊