No withholding tax code given for type & - no calculation made

What causes this issue?

You have not entered a withholding tax code for at least one of the
withholding tax (base) amounts.

System Response

If you do not enter a withholding tax code for a withholding tax type,
no withholding tax is calculated for this type in this document. The
amounts entered are therefore unnecessary, since they will not be
included in any further calculations.

How to fix this error?

Check whether you have entered a code for all withholding tax types
used for calculation and amend the code if necessary.

Error message extract from SAP system. Copyright SAP SE.