Thank you for your post!
Can you make sure the Redirect URI value that you added to your app, within step 3 is the same as within your app.
If you added more URIs within step 5 and 6, can you make sure they're also saved to your app's redirect URI list.
For more info - Redirect URI (reply URL) restrictions and limitations
Since this is a Django doc that you referenced, you can also try reaching out to the Django Community Forums.
If you have any other questions, please let me know.
Thank you for your time and patience throughout this issue.
Please remember to "Accept Answer" if any answer/reply helped, so that others in the community facing similar issues can easily find the solution.