Could not get token on sell inventory scope

[ashraf-bd]

I was trying to create a product on sandbox and production both.
So at first, I tried to use the createOrReplaceInventoryItem API where I used the scope 

https://api.ebay.com/oauth/api_scope/sell.inventory

 But while retrieving token from https://api.sandbox.ebay.com/identity/v1/oauth2/token it with grant_type -> authorization_code I got this error

{

    "error": "invalid_request",

    "error_description": "request is missing a required parameter or malformed."

}

Note: I used proper authorization header on both cases (sandbox & prod). But still getting this. Any solution?