CAN I STILL UPDATE MY iMac 21.5, Mid 2011

That will depend on what it's currently running.

The 2011 iMac cannot update beyond macOS 10.13 High Sierra.

What message exactly are you getting? If you are using the CleanMyMac app, uninstall it. It causes nothing but issues.

As Phil0124 says, 10.13.6 is tops for a 2011 Mac.

Mostly you just need a more modern Browser than Safari on High Sierra like Firefox or Brave.

https://brave.com/

Great news, take care my friend! :)