Jack,
This is an odd one. I haven’t seen any other reports of it. My best guess would be something with permissions, specifically, edit on the skuid__Page__c.IsMaster__c field. Can you verify that the user who did the install has either the Skuid Page Builder or Skuid Admin Permission Set (or both)?
As for why you can’t check the Available as a Master Page box in the Composer, it’s because the Page still has Child Pages, so we disable the check box to prevent the situation you are in right now. Obviously, something got missed though. The quickest way to get the box checked again is to query for the Page record in the Salesforce Dev Console and manually update the value to true. Does that help?
