The school has been in contact Parentpay with regards to the issue when booking free school meals. They confirmed there is a problem and they are still working on it. The issue is that when FSM meals are booked, it is asking for payment.
DO NOT PUT ANY MONEY ONTO YOUR ACCOUNT
Parentpay informed the school that parents must choose the meals, confirm their orders and then just clear the basket with the payments in - as you do not have to pay.