Mine was the same but it never finished, it just sat there for hours. Finally I tried safe mode and for a while there it looked like it was going to work but unfortunately the same thing occurred, it still didn’t complete.
I actually was so worried that I went into recovery mode and reinstalled Catalina (not a full wipe, my data is still there, I just reinstalled the macOS Catalina ). I was then finally able to boot back into macOS Catalina.
Afterwards I saved all my files and decided to try the update again but this time overnight and much to my surprise it still didn’t finish…I then force restarted the MacBook Pro and after it booted up it actually loaded into macOS Catalina.
I double checked just to see if the update actually applied and it sure did. I then restarted & shutdown the MacBook Pro several times to test it and so far everything seems good, no issues that I can tell at least but this whole experience was very buggy.