Member of Another Unit Payee

I am a stake clerk and we are looking to pay a member of another stake. There is a button in the "Add Payee" window called "Member of another unit", but it is grayed out. Is this something that would be used for this case? Why is it grayed out and is there some way to activate it? I am trying to figure out if this is a tool that could be used in this case.

Thanks.