Mark primary unit for ISO code only once

What causes this issue?

You have used the same ISO code for more than one unit of measure and
also flagged several units of measure as primary units for this ISO
code. You can only flag one unit of measure as a primary unit for one
ISO code.

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?

Flag one unit as a primary unit for each ISO code.

Error message extract from SAP system. Copyright SAP SE.