External System Web API is working fine in postman but not in Appian (Getting 500 Error)

This status code indicates that the server has encountered an unexpected condition or error that prevented it from fulfilling the request made by you. It's a generic error message that doesn't provide specific details about what went wrong, as the server may not want to disclose sensitive information to the client for security reasons.
here is what u can do to resolve this:

1. Check the HTTP method (POST, etc.) and request parameters configured in both Postman and Appian. Ensure they are consistent.
2.Confirm the content type and any other headers in your request which is required  by the API.

Check the reason phrase in your response, it seems that your request a header that is not valid,

Under Body -> ReasonPhrase in response, it says that you are sending wrong header. Please check the request.

I got the issue.

Thank you everyone.