Error during PDF form population, how do I fix and display pdf?

Question

I am trying to populate a pdf form using the example posted here:

https://www.adamalbrecht.com/blog/2014/01/31/pre-filling-pdf-form-templates-in-ruby-on-rails-with-pdftk/

I am trying to answer 2 questions..

1. What is causing the error?
2. And simply how do I execute this in my user show.html.erb?

#app/pdfs/fillablepdfform.rb

class FillablePdfForm

    attr_writer :template_path
    attr_reader :attributes
  
    def initialize
      fill_out
    end
  
    def export(output_file_path=nil)
      output_path = output_file_path || "#{Rails.root}/tmp/pdfs/#{SecureRandom.uuid}.pdf" # make sure tmp/pdfs exists
      pdftk.fill_form template_path, output_path, attributes
      output_path
    end
  
    def get_field_names 
      pdftk.get_field_names template_path
    end
  
    def template_path
      @template_path ||= "#{Rails.root}/lib/pdf_templates/EmployeeFormFillable.pdf" # makes assumption about template file path unless otherwise specified
    end
  
    protected
  
    def attributes
      @attributes ||= {}
    end
  
    def fill(key, value)
      attributes[key.to_s] = value
    end
  
    def pdftk
      @pdftk ||= PdfForms.new(ENV['PDFTK_PATH'] || '/usr/local/bin/pdftk') # On my Mac, the location of pdftk was different than on my linux server.
      Rails.logger.info @pdftk
    end
  
    def fill_out
      raise 'Must be overridden by child class'
    end
  
  end

#app/controllers/testpdfform_controller.rb

class TestpdfformController < FillablePdfForm

    def initialize(user)
        @user = user
        super()
      end
    
      protected

      def fill_out
        [:firstname, :lastname].each do |field|
          fill field, @user.send(field)
        end

      end

end

#app/controllers/users_controller.rb

class UsersController < ApplicationController

    def index
        @users = User.all
    end

    def new
    end

    def show
        @user = User.find(params[:id])
        respond_to do |format|
          format.pdf { send_file TestPdfForm.new(@user).export, type: 'application/pdf' }
        end
    end

    def new
        @user = User.new
    end

    def create
        @user = User.new(user_params)

        if @user.save
            #flash[:notice] = "Book saved"
            redirect_to @user , notice: 'User Saved'
            # flash[:notice] = "Book Saved."
            #redirect_to action: 'index', notice: 'Book Saved'
        else
            render :new, status: :unprocessable_entity
        end
    end

private
    def user_params
        params.require(:user).permit(:firstname, :lastname)
    end

end

#config/initializers/mime_types.rb

Mime::Type.register "application/pdf", :pdf