Hi
You probably need to re-format the esternal drive. You'll need to move your library somewhere else while you do it. Also take a backup just to be sure.
It must be formatted APFS or MacOS extended (journaled) (preferred for spinning drives) and not case sensitive.
It Must not be (or have been) used for time machine. (A time machine drive can be used if re-formatted)
It must be able to have the 'ignore ownership" checkbox ticked.
It should not be on a memory stick, SD or similar.
It must also not be a network drive (a direct connection is required - eg USB or Thunderbolt). File sharing services such as iCloud Drive, Drop box, Onedrive etc are not compatible with Photos libraries.
See
https://support.apple.com/en-gb/HT201517