NOT ABLE TO PAY A BILL WITH MY TOTAL BALANCE

WHY AM I NOT ABLE TO PAY MY BILL? IT SAYS THAT I HAVE $105 OF T-MOBILE BALANCE AND THAT BALANCE IS DUE ON AUGUST 12 BUT IT SAYS "LET'S TRY THAT AGAIN, ADD A PAYMENT METHOD TO PROCEED." WHY IS THIS HAPPENING? I WILL NOT PAY MY BILL UNTIL THEY FIX THIS.