_form.html.erb

<%= form_with(model: recomendacion) do |form| %>
  <% if recomendacion.errors.any? %>
    <div id="error_explanation">
      <h2><%= pluralize(recomendacion.errors.count, "error") %> prohibited this recomendacion from being saved:</h2>

      <ul>
        <% recomendacion.errors.each do |error| %>
          <li><%= error.full_message %></li>
        <% end %>
      </ul>
    </div>
  <% end %>

  <p>
    <%= form.label :descripcion %><br>
    <%= form.text_field :descripcion %>
  </p>

  <div class="actions">
    <%= form.submit %>
  </div>
<% end %>

recomendacion.rb

class Recomendacion < ApplicationRecord
    validates :descripcion, presence: true
end

recomendacions_controller.rb

class RecomendacionsController < ApplicationController
  before_action :set_recomendacion, only: %i[ show edit update destroy ]

  # GET /recomendacions or /recomendacions.json
  def index
    @recomendacions = Recomendacion.all
  end

  # GET /recomendacions/1 or /recomendacions/1.json
  def show
  end

  # GET /recomendacions/new
  def new
    @recomendacion = Recomendacion.new
  end

  # GET /recomendacions/1/edit
  def edit
  end

  # POST /recomendacions or /recomendacions.json
  def create
    @recomendacion = Recomendacion.new(recomendacion_params)

    respond_to do |format|
      if @recomendacion.save
        format.html { redirect_to @recomendacion, notice: "Recomendacion was successfully created." }
        format.json { render :show, status: :created, location: @recomendacion }
      else
        format.html { render :new, status: :unprocessable_entity }
        format.json { render json: @recomendacion.errors, status: :unprocessable_entity }
      end
    end
  end

  # PATCH/PUT /recomendacions/1 or /recomendacions/1.json
  def update
    respond_to do |format|
      if @recomendacion.update(recomendacion_params)
        format.html { redirect_to @recomendacion, notice: "Recomendacion was successfully updated." }
        format.json { render :show, status: :ok, location: @recomendacion }
      else
        format.html { render :edit, status: :unprocessable_entity }
        format.json { render json: @recomendacion.errors, status: :unprocessable_entity }
      end
    end
  end

  # DELETE /recomendacions/1 or /recomendacions/1.json
  def destroy
    @recomendacion.destroy
    respond_to do |format|
      format.html { redirect_to recomendacions_url, notice: "Recomendacion was successfully destroyed." }
      format.json { head :no_content }
    end
  end

  private
    # Use callbacks to share common setup or constraints between actions.
    def set_recomendacion
      @recomendacion = Recomendacion.find(params[:id])
    end

    # Only allow a list of trusted parameters through.
    def recomendacion_params
      # params.fetch(:recomendacion, {})
      params.require(:recomendacion).permit(:descripcion)
    end
end