Problem with upgrade to Big Sur on my Mac

I currently have an iMac, OS High Sierra, Version 10.13.6 - from mid-2014; I attempted to upgrade to Big Sur - but I got stuck just finding how/where to download the app. I went to the app, and when I clicked on ‘download’ nothing happened. If I can’t download it, how can I upgrade to it?

It says that my computer is compatible with Big Sur - then why am I have issues with it. Am I supposed to be upgrading to Catalina instead? My Mac is way behind my other 2 devices…

But - what’s the issue with Big Sur?

Thank you