Cannot upgrade from Mojave to Big Sur

Hi

I have tried all the workarounds posted on here that I could find, including terminal commands, and also followed the instructions on how to upgrade to Big Sur via the Mac App Store, but I am still unable to download Big Sur, getting the "Update not found" error message.

My machine is a 2017 Retina 5k 27" iMac, so is eligible for the upgrade, and has over 700 gig of free space. I have checked for and removed any 32-bit apps, restarted about a few times and cleared the nvram/pram.

There is a message in the Software Update tab in Settings asking me to upgrade to Monterey, but this is my work Mac and they don't want us upgrading to that yet. I cannot remove it and even though I have "Check for updates" unchecked it popped up and Software Update still checks for updates. Perhaps this is why I have an issue?

Thanks in advance.