Your VAT settings are all housed within your Business Profile.
Head to your 'More' tab
Tap on Business Profile
Follow the instructions to enable VAT
With each and every transaction we’ll give you the options you need to record VAT in the right way for your business. If the rate we suggest isn’t quite right, you can choose from a set of alternative rates that are typical for each specific transaction, or you can simply tap to see the full list and select for yourself.