User is trying to batch update the Paid flag of some expense entries, but is unable to do so.
This happens when the expense entries you are trying to update are not reimbursable in the first place. The Paid flag is only relevant for reimbursable expenses and whenever you make a payment against reimbursable expense entries in the Bill Payments screen, Core checks the Paid flag for the entries. So you must make sure that the expense entries you are batch updating are all reimbursable. You can do that by first batch updating Set Reimb flag to True for all such expense entries and then updating the Paid flag.