Im trying to change my account Default tax rate selection to No Vat but gett this error quickbooks An application error has occurred while processing your request : -90

Hi there, ashley25. 

I would recommend trying the troubleshooting steps to resolve issues with the QuickBooks Online Windows App. 

To reset the app:

1. Click on the Help menu and select Reset the app.
2. Click the Reset app.

You can also find more of them in this article: QuickBooks App for Windows and Mac: General support

In case the transaction is already included in your sales tax return, that is also the reason why you're getting this error message. This means I recommend reaching out to our support team. They have the tools to help you get past this error:

1. Click on the Help icon.
2. Select Contact us.
3. Enter a topic and click on Let's talk.
4. Click on Start a chat. 

Let us know if you have other questions.