First repayment amount can only be used for Flat Rate interest method.

For example, it is not possible to enter first repayment amount for Reducing Balance interest. The reason is that let's say you enter 1000 in first repayment amount, it is not clear whether this is principal or interest or combination of the two. Hence, we can't calculate the next repayment amount since that is calculated based on remaining principal for Reducing Balance interest. Same applies for other interest methods.