Check the commission's status badge on the Checkouts tab. Only Unpaid, Paying and Rejected commissions can be moved to Paid; commissions that are already Paid, or that the system set to Total Refund / Partial Refund because the customer's order was refunded, cannot be changed. If every selected commission is in one of those locked states the update fails with "Cant update selected comissions!"; if you selected nothing you get "You need to select at least 1!". Also make sure your role has the Affiliates sales permission – without it the request is rejected with "Invalid Permission!".