'The requested version of macos is not available - macOS Catalina
You can check if you have a non-standard update source:

defaults read /Library/Preferences/com.apple.SoftwareUpdate CatalogURL


If CatalogURL does not exist, that's OK and the standard situation to pull from the non-beta updates. To force a beta source, you write this key (as root if needed with sudo)

sudo /usr/sbin/softwareupdate --set-catalog https://swscan.apple.com/content/catalogs/others/index-10.15seed-10.15-10.14-10.13-10.12-10.11-10.10-10.9-mountainlion-lion-snowleopard-leopard.merged-1.sucatalog.gz


Choose About This Mac > Software Update. Wait a moment. An item for macOS Catalina Beta appears.



You can request an update as well from the command line just as you set the default:

softwareupdate -al


Change the lower case L to a lower case I above and prepend sudo to download and install all updates.