I’m getting the following error when trying to authenticate with my OAuth 2.0 app:

This is usually caused when the Redirect URL entered in the fmAccounting Link Organisation screen doesn’t match one of the Redirect URIs saved against your app on developer.xero.com. Make sure these are exactly the same (copy and paste helps to ensure they are identical) and try again.

The Xero API Token Management: Best Practices and Troubleshooting guide also has a lot of useful information.