I'm getting an unauthorized client error

Possible cause for unauthorized client errors:

There are several possible reasons you could be getting unauthorized_client, including:

• You have not ensured that code_challenger and code_verifier pairs are unique per request.
• You've reused an authorization code parameter in the connect/token request.
• There's a 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.