Let's go through a couple of things you need to check for you to add VAT codes on your mobile app.
Take for example an invoice. Whenever you create a new one, ensure that Sales Tax Is set to either Excluded from amounts or Included in amounts.
Another thing to watch out for is when you add a product or service. After you tap a product or service, select a VAT code under the Tax rate drop-down menu. Please be advised, this option is unavailable if you set the sales tax to Not applicable.