When we have a tax combination " Group and group Item' with a sales tax code shared between them with a Zero Rate it throughs an error that the tax combination is not valid.

This happened because the result of the combination is Zero, so the tax code validation read the combination as a wrong one.

To resolve that we need to turn the toggle for "Check sales tax code " to None or Warning.

But this is not the best option as we need the validation when we have a wrong combination to prevent users from proceed also we need to use the Zero rate so we trace which transactions were posted using this VAT code >tracing error or missing tax transactions becomes easier.

So the suggestion would be to remove the Zero tax code form the validation as long as the setup for the sales tax code, Item and group is correct it should proceed as normal with no error.