A 2011 iMac will only go to High Sierra 10.13.6, current versions of Mac OS (Mojave and later) require a GPU with Metal. This did not become available in iMacs until the 2012 models.
Before you ask, no there are no work arounds and no you cannot update your 2011. In short, if you want to get more current, it's time to buy a new Mac. Yours is 10 years old now so if it still has original internal components like the original HD the it is running on borrowed time anyway.
Finally, beginning with Catalina 10.15.x all of your appliications must be 64bit. If you run some legacy apps and want to move them forward to run on the new Mac they must be updated (and in some cases replaced) with their 64bit counterparts. Please read the following article and it will help you find the apps that need updating or replacing in order to run them in Catalina and beyond.
https://osxdaily.com/2018/03/06/find-32-bit-apps-mac/
Good luck.