PesaController

welcome_controller.rb

def order
    pesapal = Pesapal::Merchant.new(:development) #declare a variable

    pesapal.order_details = { #pass the info to order_details array
        :amount => params[:amount],
        :description => params[:description],
        :type => 'MERCHANT',
        :reference => '808-808-606',
        :first_name => 'first_name',
        :last_name => 'last_name',
        :email => '[email protected]',
        :phonenumber => '+254722123456',
        :currency => 'KES'
    }
    session[:pay_info] = pesapal.order_details
    @order_url = pesapal.generate_order_url #generate the url

    render 'pesapal'
  end


  def callback
    #declare a variable
    pesapal = Pesapal::Merchant.new(:development) 
    #@payment_status_array = session[:pay_info]
    #@reference = @payment_status_array['reference']
    #@payment_status = pesapal.query_payment_status("808-808-606")
    render plain: pesapal.query_payment_status("AtM9FlNfOi").inspect
  end

Your implementation is missing a few things (I think) ...

1. Where do you pass credentials?
2. Where do you set the callback?

I mean something like this ...

pesapal.config = {
  callback_url: "YOUR_CALLBACK_URL",
  consumer_key: "SOME_KEY",
  consumer_secret: "SOME_SECRET"
}

I think either I don't understand your use-case or you misunderstand how Pesapal works. Please read this simplified step-by-step.