Handling unit requirement is not unique

What causes this issue?

This error message appears in the following situation:
You want to post several goods movements at the same time in different
storage locations. In other words, the
<DS:GLOS.3526C023AFAB52B9E10000009B38F974>material document</> to be
posted has several items. According to the system,
<DS:GLOS.35CEB720B3CC735DE10000009B38F982>Handling Unit Management</> is
active in at least one of these storage locations. Whether an item is to
be processed using <DS:GLOS.3618AA32895864D8E10000009B38F985>handling
units</> (HU) or not, depends on the storage location.
For the goods movements you want to post, the following cases are
possible:
The first item refers to a storage location subject to a HU. One of the
following items is to be posted in a storage location that is not
subject to a HU.
The first item refers to a storage location that is not subject to a HU.
One of the following items is to be posted in a storage location subject
to a HU.
In both cases, there is a situation in the document to be posted that is
not clear.
For goods movements in a storage location that is not subject to a HU,
the system wants to create a material document.
For goods movements in a storage location subject to a HU, the system
wants to create a <DS:GLOS.3526BEB2AFAB52B9E10000009B38F974>delivery
</>.
The system, however, can only create a material document <ZH>or</> a
delivery. It is not technically possible to create two documents at the
same time. The item created first determines whether the complete
document is subject to a handling unit or not.

System Response

As the situation is unclear, the system cannot post these goods
movements.
IF &[SWITCH]S4H_DOCU_ON_PREMISE& = 'X'.

Procedure for System Administrators

You can set this message in Inventory Management Customizing under
<ZH>Define Attributes of System Messages</>. If you change the message
status to <LS>W</> (warning), the system creates a delivery. <ZH>All</>
items are therefore copied into a delivery, including those that are
usually posted with a material document. This is necessary, as at least
one item requires a delivery because of the HU obligation, and the
system can create only one type of document at the same time. It is
therefore obligatory in this case to create a delivery.
This is the only possible way of processing documents that are unclear.
A delivery is then always created for all items if at least one item
features a storage location subject to a HU.
<ZH>Note</>
Some transactions, such as the pick list (transaction CO27), call the
<DS:GLOS.3526B20FAFAB52B9E10000009B38F974>function module</>
<DS:FU.MB_CREATE_GOODS_MOVEMENT>MB_CREATE_GOODS_MOVEMENT</> to post the
goods movement. This function module only outputs error messages,
warning messages are not displayed. In this case, a delivery is directly
created without a warning message.
ENDIF.

Error message extract from SAP system. Copyright SAP SE.