My imac is on Yosemite but is telling me 'no updates available'

It can update to Catalina 10.15.7, please use the steps in How to get old versions of macOS

Please let know the model year of the computer and we can tell you the latest version it can run and how to get it. If you are unsure of the model year please click the in the upper left of the display and click About this Mac. It will tell you the model year and size.