Photos Library started a repair but stopped

Is the system version shown below your post still valid? "MacBook Pro 13", macOS 10.13"?

If you just upgraded to macOS 10.15 Catalina, the external drives you have used before may mo longer be compatible. Make sure

• that your external drive has not been used for Time Machine backups,
• that the file system format is MacOS Extended (Journaled) or APFS and not case-sensitive,
• that the drive is locally mounted and not accessed over the network.

See:

  Move your Photos library to save space on your Mac – Apple Support

Can you set the ignore ownership flag on the external volume, as described here? https://support.apple.com/en-gb/HT201517 (the checkmark at the bottom of the Info for the external volume). You can see the file system format, if you click the disclosure triangle to the left of "General" in the Info.

• If you can enable the checkmark try to repair the library on the external volume again. To be sure, that Photos is repairing the correct library, select the library on the external volume, then keep holding down the key combination ⌥⌘ while double clicking the library, not the Photos.app.
• If you cannot set the ignore ownership flag the drive is not safe to use for a Photos library and you may have the same trouble over and over again, unless you format the volume. But you have to save all your data from the drive somewhere else, because reformatting will erase it.