ISO unit of measure & is not assigned (item &)

What causes this issue?

In the specified item of the <GL:EDI>EDI</>
<GL:Intermediate_Document>Intermediate Document</> there is an ISO unit
of measure, which is not marked as primary code.

How to fix this error?

If you want the ISO unit of measure to be marked as primary code, make
the appropriate entry. Otherwise, find out how these values got into
the Intermediate Document.
Make the appropriate settings for the ISO unit of measure in
<GL:customizing>Customizing</>, if you have the authorization to do so.
If you do not have the necessary authorization, inform your system
administrator.

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.