Purchase order (PO) numbers that have been deactivated cannot be used on new or ongoing projects.
If you are creating or editing a project and want to use a PO number that was previously deactivated, you will see an error message. In case you are sure that number is the one to be used, you (or an administrator, if you are not one) have to follow these steps:
1. Go to Settings > Purchase Orders
2. On the list of Inactive Purchase Orders, find the one you want to use on your project and click the Activate link.
After activating the purchase order number, it will be available again for being used in any projects.