HU &1 is already loaded

What causes this issue?

You are attempting to repack either into or out of a handling unit that
has already been loaded, or you are attempting to create or confirm a
warehouse task in or out of a handling unit that has already been
loaded.

System Response

Repacking and other movements into or out of a handling unit that has
already been loaded are not allowed. The system issues an error message.

How to fix this error?

If the affected handling unit was loaded without a warehouse task, first
cancel the loading of the handling unit.
If the affected handling unit was loaded with a warehouse task, unload
the handling unit.

Procedure for System Administrators

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

Error message extract from SAP system. Copyright SAP SE.