High Sierra and Catalina

Try quitting the installer.

Then locate it in the Applications folder - it should be called "Install macOS Catalina" - and double-click to run.

If it still does not work, try running the installer while booted in Safe Mode.

https://support.apple.com/en-us/HT201475

Please see this link to know whether you iMac is capable of running Catalina.

Best of luck

You may have to check if your Mac is any of the following that is in the link.

Compatible Mac's

Which mac model exactly?

And how much free space on your system drive? Not enough space is the most common reason...

So your mac is compatible and has enough space. Good.

What exactly happens when you try to install Catalina?