difficulty upgrading latest Catalina 10.15.6

What version are you on now?

Are you running any VPN, Anti-Virus, or Cleaning apps?

If 10.15.x...

Safe Boot, (holding Shift key down at startup).

Try the standalone 10.15.6 in Safe Mode, but not using Firefox...

Combo…

https://support.apple.com/kb/DL2048?viewlocale=en_US&locale=en_US

10.15.5 to 10.5.6…

https://support.apple.com/kb/DL2047?viewlocale=en_US&locale=en_US

Supplement…

https://support.apple.com/kb/DL2049?locale=en_US

Any luck yet?