The requested version of macOS is not available.
I am trying to Update to Big Sur and this is the message I get when I click to get the update. I don't understand what the issue is.
MacBook Pro 13″, macOS 10.15
Is your Mac from 2013 or 2014?
There were some issues with this models and Apple is probably working on a fix.
Take a look here If you can’t install macOS Big Sur on certain 13-inch MacBook Pro computers from 2013 and 2014 - Apple Support
For more details on it, call Apple Support.