3824792 - Why can't an expense processor modify mileage expense amounts or Travel Allowance fields?

I'm trying to modify the Amount field for a Mileage expense or use the Mileage Calculator from the Expense Processor view, but I'm unable to do so. I also cannot modify the Expenses and Adjustments fields on the Travel Allowance page. What can I do?

Modify mileage expense amounts and Travel Allowance fields as an expense processor. , KBA , CON-EXP-EXP , Expenses , CON-EXP-ADM-MIL , Mileage Configuration , CON-EXP-ADM-TA , Travel Allowance , Problem