Issues at this step in the flow are commonly due to either the authorization code having expired (it is valid for 60 seconds) or issues with the Authorization header.
To set up your Authorization header, you'll need both your app's client_id (which is your API key) and your client_secret. You would put these values in the following format: "client_id:client_secret", then convert that string to Base 64 encoding. The result is used in the Authorization header. An example would look like:
Aarron G. API Support Specialist Did I answer your question? If so, please mark my post as an "Accepted Solution" by clicking the Accept as Solution button in the bottom right hand corner of this post.