Cant upgrade my Macos catalina

It may not support Catalina. You need to study macOS Catalina is compatible with these computers - Apple Support and then see if the model your iMac you own supports Catalina or not. If it is supposed to then try restarting in Safe Mode and download and install Catalina while in Safe Mode.

If you don't know how to check the model year of your iMac click the Apple symbol in the upper left of the display and then click About this Mac. It then opens a small window and right below the word iMac it will indicate the model year of the computer.

If your computer is too old to support Catalina, then it is time to replace the computer. Catalina is itself quite out of date and your computer would be at least 10-11 years old which makes it obsolete (meaning it will not run current versions of Mac OS) and vintage which means repair parts are largely not available.

You need to have a 2014 or later iMac to upgrade beyond 10.15 Catalina.

What year model is your iMac...?

see > Identify your iMac model - Apple Support