User is trying to edit a reimbursable expense entry, but gets an error that some of the properties cannot be changed as the expense entry is already paid.
This can happen if you try to change the expense entries that are already paid through the Bill Payments screen. Core checks the Paid flag for such entries and allows very limited editing for them. To edit such entries, you need to first delete the associated bill payments, make the changes to the entries and then re-create the payments. You might also want to check out this article: Marking expense entries as Paid.