I fixed it!!!
just thought I would put it here incase anyone else is wondering
basically, all I had to do was make sure the conditions were in place for the IF formula to be activated, when I input the formula.
so, if Mr Smith always pays by direct debit, and direct debit appears in my validation list, Excel let me put the If formula in as long as I made sure the row I was inputting it into was a row where Mr smith was paying by direct debit.
that was a terrible sentence, I hope it made sense