As of version 9.9.5.x, we no longer support billing with Unpost and Repost buttons, as the function will only be activated if you have purchased and bought an Access UBS Inventory & Billing System License.
Please proceed to create a Credit Note to cancel any transactions posted to accounts that require amendments. Then create a new bill for your customers and post it to Accounts.
Note: Due to limitations in the billing system, this function is not enabled. Customers must purchase an Inventory & Billing (I&B) license to activate this feature.