VAT registration number defined for company code &1 is wrong or missing

What causes this issue?

The Belgian DME formats require the VAT registration number of the
company code making the payment. This number does not exist, or has
been maintained incorrectly.

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?

Maintain the VAT registration number of the company code making the
payment.
<DS:TRAN.OB88>Proceed</>

Error message extract from SAP system. Copyright SAP SE.