Not possible to enter different order unit

What causes this issue?

You wish to use a different order unit to the one stored in the
material master record.
You cannot do this because the <ZH>variable order unit</> indicator has
not be selected in the material master record.

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.