you don't tell us what your Mac model is, so we don't know if it will accept Catalina. You might try this. I you see an installation app in the Applications folder, move it to the trash and empty it. Then restart the computer in the safe mode: hold the shift key down on restart. This can take up to 10 minutes to complete as the os is clearing out things and fixing things. Now, run Disk Utility. Repeat if necessary until the result is no problems found. Then try to complete your update and restart the computer. I hope this is helpful. If the problem persists, please let us know and other things can be tried. Good luck.
Incidentally, I am assuming that you know that any 32 bit apps will no longer work under Catalina.