I'm getting an unauthorized client error?

Possible cause:

There are quite some possible reasons why you could be getting unauthorized_client. See a list of likely causes below:

• Not ensuring that code_challenger and code_verifier pairs are unique per request.
• Reusing an authorization code parameter in the connect/token request.
• Mismatch between the redirect_uri parameter in the connect/authorize endpoint and connect/token requests.

See screenshot below : 

Steps To Resolve:

• Ensure you're making use of a url that is whitelisted for your client. You may refer to your client credential JSON file for a list of valid redirect_uris. For more information you may refer to this article.
• Ensure that the redirect_uri used during the connect/authorize request matches that used in the connect/token request.