Step 4 exchange auth code for access and refresh token

[NiLM]

I keep receiving an error with my fit bit request specified in step 4 of the authorization guide. 

I get userCode from the call back url. 

const sendJson = {

                "method": "POST",

                headers: {

                    Authorization: 'Basic MjNCSFM4OmM0ZTI2Mjg0MjIzOWQwNWQzZjljNzU2ZGExZDAzZmUw',

                    'Content-Type': "application/x-www-form-urlencoded"

                }

            };

            var requestUrl = `https://api.fitbit.com/oauth2/token?clientId=23BHS8&code=${userCode}&grant_type=authorization_code`;

            fetch(requestUrl, sendJson)

                .then(response => response.json())

                .then(data => console.log(data));

In postman my error returned is 

    "errors": [

        {

            "errorType": "invalid_grant",

            "message": "Authorization code invalid: cff86e02c5c4c9f5bdbf3253c1c675661f3b16d4 Visit https://dev.fitbit.com/docs/oauth2 for more information on the Fitbit Web API authorization process."

        }

    ],

    "success": false

}

Not sure why i am getting this error.