Update not found

Getting the exact same thing here, unfortunately have not found a solution as of yet.

I'm running into the exact same issue have had no luck with work arounds. I've seen the workaround below recommended in different forums, but I haven't had any luck with it. If you happen to find a fix, please let me know.

open terminal
sudo -i
enter root password
/System/Library/PrivateFrameworks/Seeding.framework/Versions/A/Resources/seedutil

Unenroll from the Beta Program 
/System/Library/PrivateFrameworks/Seeding.framework/Versions/A/Resources/seedutil unenroll

Enroll in the DeveloperSeed Beta 
/System/Library/PrivateFrameworks/Seeding.framework/Versions/A/Resources/seedutil enroll DeveloperSeed

Close and re-open System Preferences > Software Update
The new Beta should now show up

Been trying to load Bug Sur for a week .... I get the same "The requested version of macOS is not available." 

Ugh