VAT registration number of customer number &1 cannot be changed

What causes this issue?

If a customer number has been used in documents with a Portuguese
digital signature (in a Portuguese company code or in a Portuguese
organizational unit of a foreign company code), the VAT registration
number can be changed only if it was previously empty or it contained a
generic value. This restriction applies to all changes, including the
correction of errors.

The system issues an error message and will not allow you to continue with this transaction until the error is resolved.

System Response

The system prevents the change.

How to fix this error?

To use the VAT registration number that you entered, create a new
customer master record.

Procedure for System Administrators

Error message extract from SAP system. Copyright SAP SE.