The only way you can find the solution to this problem is by accessing the Help menu embedded into the application. The official website doesn't offer any information related to invoice and customer management. In this case, the only thing I can recommend is contacting the developer of the application. By default, if you are an administrator, you should have the possibility to unlock the invoice. Try to right click it where it's displayed then analyze the available options. Eventually, access the properties menu and then perform the unlock.