Problem when trying to add instance with Action Builder...

I've done this a million times, but apparently I'm forgetting something. I make a new document, add a table that is 2x2 (Table1), add a button (Button1), go to the action builder. My condition is "When button Button1 is clicked" and my result is  "Add a new instance of Row1", it tells me "This subform is not configured to be repeatable". What am I missing!!!!!!!!