QuickBooks requires that when using it's Single Sign On process in a 3rd party app, that the users' email address must be verified. The steps to do this are as follows:
- Navigate to https://accounts.intuit.com.
- Log in with your QuickBooks user.
- Navigate to the Sign in & security menu.
- Next to email address, there should be a link to verify. Click the link and complete the process.
- QuickBooks will send an email to that email address with instructions to complete the verification.