The app won't let me save any new invoices that require the 20% vat reversal. Does anyone know how to fix this?

It's good to see you here in the Community, @ross26.

Let me guide you on where you can successfully save your invoice that contains the 20% VAT reversal.

When creating invoices for CIS transactions, you'll need to do this on your browser since the app is a simplified version of the program. Thus, some features are not available on the said platform. Just log in to your account on this site qbo.intuit.com and generate the invoice from there.

In addition, here are some resources that you can utilize to give you more insights about the new VAT code for CIS together with its commonly asked questions:

VAT Reverse Charge For Building and Construction Services (CIS)

What do I need to add to my invoices?

Let me know if you need further help with creating invoices or with QuickBooks. I'll be right here to assist you. Keep safe!