Graphql Unknown argument on field

looks like you are missing args on users, hence, it should look like this:

var users  = new graphql.GraphQLObjectType({
  name : 'user',
  description : 'this is user info',
  fields : function(){
    return {
      id :{
        type : graphql.GraphQLInt,
        resolve(user){
          return user.id;
        }
      },
      username :{
        type : graphql.GraphQLString,
        resolve(user){
          return user.username;
        }
      },

      posts:{
        args: {
        id:{
            type : graphql.GraphQLInt,
        },
        type: new  graphql.GraphQLList(posts),
        resolve(user, args){
          // Code here to use args.id
          return user.getPosts();
        }
      }


    }
  }
});