App not allowing the option to add vat to an invoice, desktop version still working.

Let's perform some troubleshooting steps so you'll be able to add VAT to your invoice using the mobile app, steve251.

To begin with, let's clearing the app data to refresh the system and eliminate any outdated files that may lead to unexpected behaviors.

I recommend clearing the app data to refresh the system and eliminate any outdated files that may lead to unexpected behaviors.

If you're using iOS

1. Go to Settings, then tap General.
2. Tap iPhone Storage, then tap the QuickBooks Online app.
3. Tap Offload App.

If you're using Android

1. Go to Menu ☰, then tap More Options ⋮.
2. Tap Settings, then Refresh Data.
3. Tap YES to confirm.
4.  

This will free up the storage without deleting your app's documents and data.

Once done, attempt to add a VAT to your invoice. However, if the issue persists, I suggest uninstalling and reinstalling the application to ensure it's the latest version and reset the program to its default settings.

Alternatively, you can manage your invoices through the web version of QBO.

Let us know in the comment below if you have a follow-up questions about managing your invoices. We're always here to provide assistance.