ISO code & is not assigned to a country key

What causes this issue?

No internal code assignment was found for the ISO country code &v1&.
Either the internal code does not exist, or it has not been assigned to
the 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?

Please assign an internal code for the ISO code.

Error message extract from SAP system. Copyright SAP SE.