Today, I decided to install the Catalina Beta but after enrolling to the beta program, I encoutered this issue:

The requested version of macos is not available

The solution I found is to execute these two commands in the Terminal:

sudo defaults read /Library/Preferences/ CatalogURL

sudo /usr/sbin/softwareupdate --set-catalog

After you execute the commands, reopen the Software Update section in the Preferences and install the Beta.