I upgraded to Catalina and ran into trouble when I tried to import my photos FROM my iPhone 7 to my Mac. I already had my phone connected so I could sync things through the new Finder window (which worked fine). When I opened Photos to sync pictures as well and clicked on my iPhone in the Devices list, it immediately showed a message that said "Preparing to import from…" and then just sat there with no indication anything was happening. After several searches for help, I finally figured out something that worked.
I closed Photos on my Mac, "ejected" my iPhone via the Finder window and disconnected it physically, reopened Photos (without my phone connected yet), woke my phone up/turned it on and physically connected it. The Photos devices list recognized my iPhone and blinked through a message about granting permission, then popped up with the photos from my phone to allow me to sync/download them. Hopefully this will help someone else if they have the same problem.