If the message 'Amendment cannot be added as existing allowance for this person has a later start date' is received, an existing allowance is present for the same start date or end date. To resolve this, follow the steps below.
Cancel the existing allowance that has the same start date as what is needed, use an end date that is the same as the start date (or else today's date)
Add a new allowance by using a start date of the day after the start date of the existing allowance that is not cancelled, to set up another allowance to end on the new end date.