Last movement of handling unit & is not yet confirmed

What causes this issue?

<GL:handling_unit>Handling unit</> &V1& is currently included in an
open <GL:transfer_order>transfer order</>. Therefore, it cannot be
moved.

System Response

The system issues an error message and will not allow you to continue with this transaction until the error is resolved.

How to fix this error?

Confirm the open transfer order.

Error message extract from SAP system. Copyright SAP SE.