Hi! I have been trying to set up an API that will extract activity data based on dates.
For Step 6, the checkboxes shown will vary based on the scopes you request. I believe since the original documentation was written Strava has added additional scopes so the screenshot no longer matches the URL. Scopes are listed @ https://developers.strava.com/docs/authentication/#detailsaboutrequestingaccess. To get an equivalent set of checkboxes to the image you would need read and activity:read, so your URL would be:
As to the 404 error, I'm not sure what to tell you on that one. I just tested it locally with postman and it worked fine for me. In case it helps, the exact steps I followed:
One other thought, although I don't think it matters for this step, is to edit your app (in Strava) to have its Authorization Callback Domain set to localhost.