Error in Photo application

As a test launch Photos with the Option key held down and create a new, test library.  Import some photos and test to see if the same problem persists. Does it? This tells us if the problem is limited to your current library or is more wide spread.

Then open your current library and try importing the same photos again. Any improvement?

How long have you waited for the upgrade to finish, Patricia? It can take several hours for a large library. And the library needs to be on a volume with plenty of free storage.

Where ist the Photos Library stored? And how much free storage is in that location?

Which system version is running on your Mac? And how oldie the library in question, that needs upgrading?

Photos may be hanging when upgrading an old library, if the library contains videos or image files in a format, that is no longer supported on your current system version.