Output sets and the TMS checkbox

Guys, I’m stuck.

I’m in Output Set Builder, I want to check the TMS check box. Nothing is happening.

I have looked through all manner of places trying to find clues, but alas I have not found anything that enables this. Other OPS have it checked already.

What obvious clue am I missing here? How can I make that pretty green tick come alive in other OPS? 

Thank you!

Natasha