Confirm the open warehouse task for HU &1

What causes this issue?

You may not create, confirm, or cancel a warehouse task (WT) from
handling unit &V1&.
There is already an open handling unit WT for handling unit &V1&.
Handling unit &V1& and the current WT reference an inbound delivery.
In order that the system can update the document flows for the affected
inbound deliveries correctly, you must confirm or cancel the handling
unit WT for HU &V1& before you can create, confirm, or cancel the
current WT from handling unit &V1&.

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?

Procedure for System Administrators

Error message extract from SAP system. Copyright SAP SE.