Postman API Invalid Content-Type Header

I'm trying to test and API call for the Smartsheet API in Postman, but I keep receiving the same error. Even though I am defining the correct Content-Type as per the API documentation, the response I get is the error 1124, which is an invalid content-type header. I haven't been able to figure out exactly what is causing the issue. I have tried typing the header in the address and in the header tab in Postman, but neither option has given me the result I wanted.

Postman Screenshot

If I run the same request in Postman as your screenshot shows (except using my sheet ID), with the same headers that you're using, my request succeeds. i.e., if you're sending the Content-Type header with value application/json for this request, you should not be receiving the error message that you're reporting (1124 - invalid content-type header).

You would, however, receive that error message if you were either not including the Content-Type header at all OR if you were specifying an invalid value for the Content-Type header. A suggestion for troubleshooting: use Fiddler (or a similar tool) to examine the request that's being sent over the wire when you execute this request in Postman -- does it include the Content-Type header, and if so, what's the value of that header?

UPDATE:

Thanks for adding a screenshot of the body you're setting in Postman -- I believe that's the source of your issue. i.e., your Content-Type header says that the request body is in JSON format, but you're actually not sending JSON. To fix this: instead of specifying key/value pairs in Postman, select the raw radio button and specify the body in JSON format. Here's a screenshot of what that looks like: