Apple Event: May 7th at 7 am PT
I have this problem as well. Just upgraded to Catalina from 10.13 High Sierra. Wish I hadn't now.
Photos worked fine in 10.13. My early photos came from iphoto on a Mac Mini whereas I now use an iMac - but those photos were converted into Photos without a problem several years ago.
I have two user ids though. "Bob from mac mini" and "Robert xxxxxxx". I wonder if this could have contributed to the problem I now face.
iMac Line (2012 and Later)
How long has photos been stuck at 99%? The upgrade may take a very long time for a large library.
Are you using your Photos Library from both your user accounts?
Where is the Photos Library stored? Is it in your Pictures folder or on an external volume?
Catalina has more rigid security measures than the previous system versions.
For example, Photos will not be able to upgrade the library successfully, if the library is on an external volume, that has been used by Time machine. The library needs to be on a volume with a wired connection. See: Move your Photos library to save space on your Mac - Apple Support
Photos may also be stuck with upgrading the library, if the library contains media files with a codec, that is no longer supported by macOS 10.15. Catalina. While you have been using iPhoto you may have imported images and videos with an outdated codec, that no longer can be read by Photos. Such items should have been removed or converted before the upgrade to catalina.
How long has photos been stuck at 99%? The upgrade may take a very long time for a large library.
Are you using your Photos Library from both your user accounts?
Where is the Photos Library stored? Is it in your Pictures folder or on an external volume?
Catalina has more rigid security measures than the previous system versions.
For example, Photos will not be able to upgrade the library successfully, if the library is on an external volume, that has been used by Time machine. The library needs to be on a volume with a wired connection. See: Move your Photos library to save space on your Mac - Apple Support
Photos may also be stuck with upgrading the library, if the library contains media files with a codec, that is no longer supported by macOS 10.15. Catalina. While you have been using iPhoto you may have imported images and videos with an outdated codec, that no longer can be read by Photos. Such items should have been removed or converted before the upgrade to catalina.
Been stuck for an hour now. Only "Bob from mac mini" has a Photos Library. "Robert xxxxx" doesn't. By the way, my wife has a separate Photos Library on the same iMac and hers is fine.
Both our Photos Libraries are on the iMac hard disk. There may well be imported videos in my Photos Library.
Photos library repair stuck at 99%
