If there's a group there cannot be a VAT/Sales tax Code

Cause

You are trying to enter a VAT code on a row that contains a Group

In the VAT Codes table, a Group will totalize the VAT codes that pertain to the same group, therefore there must not be a VAT code in that row.
 

Solution

You must cancel the Group in the row where the VAT code is entered.

Related topics

•  VAT Codes table