SAP Note 18410 - Withholding tax field not present in credit memo

Symptom:

No withholding tax amounts (withholding tax base amount) can be entered when you enter a credit memo. Can also occur when entering an invoice (general transaction MR01).

Cause and prerequisites

The field status selection does not display the fields for the involved posting keys 21 (credit memo) and 31 (invoice).

Solution
Call transaction OB41 and change the field status (see button) for posting keys 21 and 31 for taxes: set the withholding tax fields to "Ready for input".