Hi @phildes,
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.
In case you encounter other issues on your QuickBooks Online mobile app, view this article: Troubleshooting Quick-Start Guide for the QuickBooks Online app.
Post a reply if you need further assistance with your mobile app, and I'll handle it for you as soon as I can.