Cannot retrieve LWA code with sellerId and mwsAuthToken for SP-API, access denied


I want to migrate our current application to hybrid application.
I try to use seller id and mwsAuthToken to get the LWA code in order to exchange for the LWA refresh_token where I found on the amazon migration guide. LINK

This is my request[sellerId]&developerId=[developerId]&mwsAuthToken=[mwsAuthToken]

what I got from response is

    “errors”: [


            “message”: “Access to requested resource is denied.”,

            “code”: “MissingAuthenticationToken”




Not sure what I really missing? Can anyone help about this?