Make sure that you are setting grant_type, code and redirect_uri as query parameters instead of in a JSON object:
Here you’re using redirect_url (ending with an L) whereas the parameter is redirect_uri (ending with an i). I just want to make sure that’s known in case that’s causing any issues.
Also, please make sure that URL is accessible publicly and working as expected. When I navigate to that page, I see an error.
If that doesn’t help, please send an email to email@example.com with a link to this thread. In that email, please include the full request that you’re making - including the request body as well as the response that you’re seeing.