My iCloud drive misrepresents current OS

Sorry, your original post said "my iCloud drive says I am on Mojave but my iMac is on Catalina?" -- i.e, Mojave, not Yosemite or I might have been quicker to identify this.

See the following screenshot from my Mac running Big Sur:

Note the words "or later" in that message.

It is a generic message that has confused quite a few people and could probably be better worded. Another way (maybe not better) to say that, is if your Mac is running Yosemite or later, expect to see it listed below but if you are signed in and running a Mac that is running something earlier than Yosemite, it will not be listed below (even though there is no problem with it).

Your Mac is running Catalina which is later than Yosemite and therefore you should expect to see it included in the list of devices, an you do.

Where are you seeing that (on your iMac, on icloud.com, etc -- and exactly where within that)?

Please be more specific -- "on my iCloud Drive" could be in a variety of places as could be a "devices section."

My first assumption was www.icloud.com and then going to "My Devices" in Account Settings there, but that does not display the OS for any devices that I have ever seen.