In the following event (you should make them both subevents of the button trigger), you can make an Else event. The Else event will only run if the previous event did NOT run (meet conditions). So if none of the spots checked in the first loop were valid, stop loop would never have run, and the following event will run. If the first loop did execute, then the else event would not.
But again, you have to define clearly in what order you want to check available positions. For example, why would it go to D4 and not C5? Would it go to F2 if there was a spot there when you pressed left?
Don't worry too much about checking X amount of columns, loops can do wonders :)
Edit: Also where would it go if there was a spot at F5 and F7?