Trouble Migrating MWS API to Selling Partner API

Hello There, I am migrating mws API to SP API. As per the documenation, I have tried to get the Authorization code that I cna use to get refresh token and subsequently access token for accessing SP API. I and referring this documentation https://developer-docs.amazon.com/sp-api/docs/authorization-api-v1-reference and using the API GET /authorization/v1/authorizationCode passing sellingPartnerId, developerId and mwsAuthToken.

But it is giving below error "errors": [ { "message": "Access to requested resource is denied.", "code": "MissingAuthenticationToken" } Any help appreciated. Thanks in advance.