Add a flash error where no available shipping methods to the customer

+ update specs as well

app/controllers/split_checkout_controller.rb

@@ -23,6 +23,8 @@ class SplitCheckoutController < ::BaseController
     redirect_to_step_based_on_order unless params[:step]
     check_step if params[:step]
     recalculate_tax if params[:step] == "summary"
+
+    flash_error_when_no_shipping_method_available if available_shipping_methods.none?
   end
 
   def update
@@ -47,6 +49,10 @@ class SplitCheckoutController < ::BaseController
 
   private
 
+  def flash_error_when_no_shipping_method_available
+    flash[:error] = I18n.t('split_checkout.errors.no_shipping_methods_available')
+  end
+
   def clear_invalid_payments
     @order.payments.with_state(:invalid).delete_all
   end