Add flash when order cannot be changed

app/controllers/spree/admin/adjustments_controller.rb

@@ -30,6 +30,9 @@ module Spree
       end
 
       def skip_changing_canceled_orders
+        return unless @order.canceled?
+
+        flash[:error] = t("admin.adjustments.skipped_changing_canceled_order")
         redirect_to admin_order_adjustments_path(@order) if @order.canceled?
       end
 

config/locales/en.yml

@@ -366,6 +366,8 @@ en:
     rename: "Rename"
 
   admin:
+    adjustments:
+      skipped_changing_canceled_order: "You can't change a cancelled order."
     # Common properties / models
     begins_at: Begins At
     begins_on: Begins On