User is trying to update the Cost Amount of an expense entry by changing the Cost Rate value, but cannot and gets an error that the expense is already paid.
This can happen if the expense is reimbursable or part of a vendor bill and you have already made a payment against it in the Bill Payments screen. Core checks the Paid flag for such expenses and does not allow you to change any of its properties that might affect the accounting of the bill payment. You need to delete the associated bill payment before you can update the cost on the expense entry. Check the Core Help for details on the Paid flag.