Unit of measure must be base unit, WM unit, or unit of issue

What causes this issue?

The unit of measure you have entered for loading equipment quantity is
not the base unit of measure, nor is it a WM unit of measure, nor is it
a unit of issue.

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?

Choose one of these three units of measure as the unit of measure for
your loading equipment quantity.

Error message extract from SAP system. Copyright SAP SE.