graphql 400 bad request - Must provide an operation

The problem is in the error. It says

Must provide an operation

what you're doing is executing the type definition. That's like executing a function interface definition rather than the function itself in typescript. As an example, I have a type definition like

const Comment = new GraphQLObjectType({
  name: "Comment",
  fields: {
    id: { type: new GraphQLNonNull(GraphQLID) },
    content: { type: new GraphQLNonNull(GraphQLString) },
    email: { type: new GraphQLNonNull(GraphQLString) },
    createdAt: {
      type: new GraphQLNonNull(GraphQLString),
      resolve: (source) => source.createdAt.toISOString(),
    },
    postId: { type: new GraphQLNonNull(GraphQLID) },
  },
});

followed by this query

 commentMainList: {
      type: new GraphQLList(new GraphQLNonNull(Comment)),
      resolve: async (source, args, { pgApi }) => {
        return pgApi.commentMainList();
      },
    },

and then in postman I can do

{
    commentMainList{
        id
        postId
        email
        content
        createdAt

    }
}