Can’t get off of #OSXSierra Public Beta <After Updating System Preferences> and Un-enrolling from beta.apple.com?
If anyone else has this problem:
1) Open Terminal
2) Sudo to Root User
3) “softwareupdate — clear-catalog”
Example : Use dash dash before clear <See below>
4) *Might need to delete any Install files under Applications
If you need to enable root access, this works: https://www.maketecheasier.com/enable-root-user-mac/
Worked for me.