I am trying to update my MAC operating system with no luck

What model and year is your iMac? A mid 2014 Mac or newer is required to install Big Sur.

Back up your data, check your applications for compatibility, and depending on which Mac model you have, click on El Capitan, High Sierra, Mojave, Catalina, or Big Sur. They're full-version upgrades and won't appear in the Updates list.

If you encounter a specific issue, describe it in this thread.

(204534)