How to set up error messages of associated model?

Hello rails community!

I have booking_post model that has_many reservations.

class BookingPost < ApplicationRecord
    has_many :reservations, dependent: :destroy
end

All reservation belongs_to booking_post and have some validations

class Reservation < ApplicationRecord
    belongs_to :booking_post
    before_save { self.email = email.downcase }
    VALID_EMAIL_REGEX = /\A[\w+\-.]+@[a-z\d\-.]+\.[a-z]+\z/i
    validates :email, presence: true, length: { maximum: 255 },
                    format: { with: VALID_EMAIL_REGEX }
    validates :name, :email, :phone_number, :start, :end, presence: true
end

My routes are next:

resources :booking_posts do
  resources :reservations, only: [:new, :create]
end

Methods:

class BookingPostsController < ApplicationController
  def show
    @booking_picture = @booking_post.booking_pictures.build
    @booking_pictures = @booking_post.booking_pictures
    @reservation = @booking_post.reservations.build
    @reservations = @booking_post.reservations
  end
end


class ReservationsController < ApplicationController
  def new
    @reservation = Reservation.new
  end
  def create
    @booking_post = BookingPost.find(params[:booking_post_id])
    @email= User.where(admin: true).first.email
    @reservation = @booking_post.reservations.build(reservation_params)
      if @reservation.save
        @saved_reservation = @reservation
        redirect_to :back 
        flash[:notice] = 'Reservation was successfully created.'
        ReservationMailer.fresh_message(@saved_reservation, @email).deliver_now
      else
        redirect_to @booking_post
        flash[:info] = @reservation.errors.full_messages do |m|
          m
        end
      end
  end
end

I would like to create on booking_posts/show.html.erb form_for @reservation, and render on this page errors for @reservation. When I create valid @reservation, I see on booking_posts/show.html.erb successfull flash message, but unvalid @reservation appear without any error flash messages.

form_for @reservation on booking_posts/show.html.erb:

<div class="card-action">
  <%= form_for([@reservation.booking_post, @reservation], html: {multipart: true}, class: "col s12") do |f| %>
  <% if @reservation.errors.any? %>
    <div id="error_explanation">
      <h2><%= pluralize(@reservation.errors.count, "error") %> prohibited this post from being saved:</h2>

      <ul>
      <% @reservation.errors.full_messages.each do |message| %>
        <li><%= message %></li>
      <% end %>
      </ul>
    </div>
  <% end %>

  <div class="col s6">
    <%= f.label :start %>
    <%= f.date_field :start, placeholder: "start time",  class: "datepicker" %>
  </div>
  <div class="col s6">
    <%= f.label :end %>
    <%= f.date_field :end, placeholder: "end time",  class: "datepicker" %>
  </div>
  <div class="col s6">
    <%= f.label :reservation_time %>
    <%= f.time_field :reservation_time, placeholder: "time", class: "timepicker", id: "timepicker", type: "time" %>
  </div>

  <div class="input-field col s6">
    <%= f.label :name %>
    <%= f.text_field :name, class: "validate" %>
  </div>
  <div class="input-field col s6">
    <%= f.label :email %>
    <%= f.text_field :email, class: "validate" %>
  </div>
  <div class="input-field col s6">
    <%= f.label :phone_number %>
    <%= f.text_field :phone_number, class: "validate" %>
  </div>

  <div class="waves-effect waves-light btn">
    <%= f.submit t(:submit_reservation)%>
  </div>  

<% end %>
<br>
</div>

I would like render error messages for @reservation on @booking_post page (in booking_post_path, not in new_reservation_path or anyting else). How can I do so?

Thanks for solutions