iMac stuck on setting up after installing Catalina
Imac 2017 stuck on setting up screen after update to Catalina
You may have an issue with your network or a pre-existing condition on your iMac.
I would recommend letting it run for a few more hours and if the problem persists, try a hard shutdown and then resetting the NVRAM per the directions in https://support.apple.com/en-us/HT204063