I was having the exact same error when attempting to update to the latest iOS 16 beta. I went under settings > General > iPhone storage, and then deleted the iOS beta update download there, restarted my Phone and tried the update again and it worked perfectly. Downloaded and installed successfully. Hopefully this works for anyone having the same issue.

Yep; same issue here. I can install the Big Sur beta on my 2019 MacBook Pro, but not on my new 2020 iMac. Keep getting requested version is not available message.

Double checked and Secure boot is at full security

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