The line is probably open on either a pick ticket or a preparation plan. 

Hi Paul,

As Stephen indicated, the normal reason for this error is a pick ticket includes SO lines from the order. I just saw that after my screen refreshed. You can access the pick ticket or review the preparation plan and deselect the lines. If the pick ticket is not in 'deliverable' state, you may delete the pick ticket entirely. This is a decision you make depending on whether other orders are on the pick ticket that you can sacrifice or re-pick.  In some cases where there is a customization or older patches, there could be some orphan records in the pick ticket table. In this case you will want to review with your local support team or business partner for a solution.

Stephen Thank you so much.