Where is your Photos Library stored?
If it is on an external volume, the volume needs to be prepared as described here: Move your Photos library to save space on your Mac - Apple Support
Catalina is checking the suitability of an external volume more rigidly than the previous versions of Photos. The library must not be on a volume that hsa been used for Time Machine backups or has third-party backup software installed. The volume must not be on a network share, a NAS, or a Time Capsule.
If you cannot set the "Ignore Ownership on this volume" flag as described in "Move your Photos library to save space on your Mac - Apple Support", you must not keep a Photos Library there.
Or is your Photos Library in your Pictures folder?