I added a new hub variable and defined it as type Boolean and set as False. Then I created a Button Controller so that when a button is pressed, it turns the variable True. This all worked. I then went back into Settings/Hub Variables and clicked on the true tag next to this new variable. I got the following error:
Not sure I can help, but it may be useful for others to know the platform version you are running. I'm having trouble reproducing it on the latest beta release.