Enter a valid country code for partner VAT registration number

What causes this issue?

The first two places of the partner VAT registration number are not
identical to the ISO country code of the EU country.
A VAT registration number always starts with the ISO country code of the
EU country as its first two places. The only exception here is Greece,
for which the code <ZK>EL</> is specified instead of the ISO country
code <ZK>GR</>.

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?

Enter a valid ISO country code for the first two places of the partner
VAT registration number.

Procedure for System Administrators

Error message extract from SAP system. Copyright SAP SE.