You are trying to do use -t with the API key you get from Cloud.
-t is the auth token passed to the server for Alter operation of the schema file. If used with --slash_grpc_endpoint, then this should be set to the API token issued by Slash GraphQL.
For this, you can login on http://dgraph-alpha-url:alpha-port/admin. This returns accessJwt and refreshJwt. You can use this accessJWT in -t. That should solve the problem.