Just had the same problem with the Postman test, and fixed it in Postman too:

1. Go back to the "Variable names surrounded by {{}}" in the Configure New Token section.

2. Hover over the variables you've entered, eg {{scopes}} (note: they're likely orange or red at this point).

3. While hovering, click 'Add to' in the dropdown (that appears appears from hover)

4. Select "E Environment" (assuming you saved these original details in OAuth 2.0 Environment).

5. The variables should now have turned blue.

Try 'Get New Access Token' from here again.