post_mailer.rb

class PostMailer < ApplicationMailer

  def new_post_email(user_id)
    puts "MAILER!!"
    @user = User.find_by_id(user_id)
    mail(to: @user.email, subject: 'New question has been asked!')
  end
end

posts_controller.rb

 def create
    @post = current_user.posts.build(post_params)
    
    if @post.save(post_params)
      flash.now[:success] = "Post successfully created"
      redirect_to post_path(@post)
      PostsWorker.perform_in(30.seconds, User.first.id)
    else
      render "new"
    end
   
  end

posts_worker.rb

class PostsWorker
 include Sidekiq::Worker

  def perform(user_id)
    puts user_id + "asdsad"
    puts "I am worker!"
    PostMailer.new_post_email(user_id)
  end
end