Ruby on Rails form error not giving any output/ not working

Hello guys I searched everywhere on how to output form errors but no luck. I tried almost all the code I found on the internet but still didn't work.

Here are some of my files:

view/books/new.html.erb

<div class="container">
  <h2>Create a new book</h2>
  <%= form_with model: @book, url: create_new_book_path do |f| %>
    <% if @book.errors.any? %>
      <ul>
        <% @book.errors.full_messages.each do |msg| %>
          <li><%= msg %></li>
        <% end %>
      </ul>
    <% end %>

    <div class="form-group row">
      <%= label_tag(:title, "Title:", class: "col-sm-2 col-form-label") %>
      <div class="col-sm-10">
        <%= f.text_field :title, class: "form-control" %>
      </div>
    </div>
    <div class="form-group row">
      <%= label_tag(:price, "Price:", class: "col-sm-2 col-form-label") %>
      <div class="col-sm-10">
        <%= f.text_field :price, class: "form-control" %>
      </div>
    </div>
    <div class="form-group row">
      <%= label_tag(:subject_id, "Subject:", class: "col-sm-2 col-form-label") %>
      <div class="col-sm-10">
        <%= f.collection_select :subject_id, @subjects, :id, :name, class: "form-control" %>
      </div>
    </div>
    <div class="form-group row">
      <%= label_tag(:description, "Book description:", class: "col-sm-2 col-form-label") %>
      <div class="col-sm-10">
        <%= f.text_area :description, class: "form-control" %>
      </div>
    </div>
    <%= submit_tag "Create book", class: "btn btn-success" %>

    <%= link_to "Cancel", books_path, class: "btn btn-danger" %>
  <% end %>
  <br/>
</div>

books_controller.rb

class BooksController < ApplicationController
  def index
    @books = Book.all
  end

  def show
    @book = Book.find(params[:id])
  end

  def new
    @book = Book.new
    @subjects = Subject.all
  end

  def create
    @book = Book.new(book_params)
  end

  def book_params
    params.require(:book).permit(:title, :price, :subject_id, :description)
  end

  def edit
    @book = Book.find(params[:id])
    @subjects = Subject.all
  end

  def update
    @book = Book.find(params[:id])
    if @book.update_attributes(book_params)
      redirect_to action: "index"
    else
      @subjects = Subject.all
      render action: "edit"
    end
  end

  def destroy
    Book.find(params[:id]).destroy
    redirect_to action: "index"
  end
end

routes.rb

  get 'books/new', to: 'books#new', as: 'new_book'
  post 'books/create', to: 'books#create', as: 'create_new_book'

view/layouts/application.html.erb

<main role="main">
  <%= yield %>
</main>

I don't really know what I'm missing to get the errors, grateful for your answers!!

try putting them in a flash like this and render it in your view

  def create
    @book = Book.new
    if @book.update(book_params)
      flash[:notice] = 'success'
      redirect_to action: 'index'
    else
      flash[:alert] = @book.errors.full_messages.join(', ')

      redirect_to action: 'index'
    end
  end

and in your view render those flashes like

<% if flash[:notice]%>
  <span class='notice'><%= flash[:notice] %></span>
<% end %>
<% if flash[:alert]%>
  <span class='alert'><%= flash[:alert] %></span>
<% end %>