Error when adding a serial number to a sales order line


I get the following error when trying to add a serial number to a product on a sales order line.

The sales order has a reference to a purchase order which was created from the sales order.

The warning message i receive tells me to use the inventory button to remove the marking on the order line but when I navigate to Inventory > Marking there is no possible action to make.

What could cause this problem and how can it be fixed? I am thankful for any type of advices I can get