This article explains why sometimes Kitchen Cut won't allow you to delete an item, and how to go about troubleshooting.
Kitchen Cut has certain limiting security measures, to prevent small changes from having big consequences for your data. In the case of products, subrecipes and recipes, the system will prevent you from deleting an item if it is used (or referenced) somewhere else.
So for example, if a product, lets say Carrots, are used in a recipe for, say, Carrot Soup, then the system won't let you delete the Carrot product without first removing or substituting the item from the Carrot Soup. And this logic applies elsewhere. If a Recipe is used in Menu, or reference in a Menu Item then it can't be deleted until you either remove it from or delete the Menu/Menu Item first.
To tell if this is the case the system has some indicators. If an item is 'used' elsewhere (and therefore unable to be deleted) it will have a small red line under the selection box on the left-hand side. We can see this here with a product example. Notice the Bramley Apples have a red line underneath, but Alfalfa and Baking potatoes don't. This means we could delete the Alfalfa and Potatoes, but to delete the Bramley Apples we'd first need to work out where they're in use, and remove them from or delete these records.
In the case of products, this is fairly simple - you can either edit the product and then look at the menu items it is linked to, or we can simply select the knife and fork logo (highlighted above) at the top of the page to see a list of which recipes the ingredient issued in:
You can then either remove the ingredient from these recipes, or delete the recipe altogether. You'll then be able to delete the product.
This option works for Subrecipes too, but it can be more tricky to tell why a Recipe can't be deleted. You will still see the red line listed under the recipe (as below), but there's no Knife and Fork option. In this case there's 2 possibilities: the recipe is either linked to a Menu Item in the POS section, or it is linked to a Menu in the Menu Costing > Menus section. 
In this case, the best option is to check both sections for any records that might contain the recipe you need to delete. In this case, if I wanted to delete the Apple Pie recipe, I'll go to POS > Menu Items and search for 'Apple Pie'. Sure enough, there's a menu item there. If you edit this you'll be able to confirm that it uses the same 'Apple Pie' recipe that I'm trying to delete. 
In this case I simply want to delete this recipe altogether, so I'll choose Actions > Delete on the Menu Item to delete this first, but you may want to just Edit and swap out the linked recipe for a different one - this would work too.
Once the Menu Item is delete, if we go back to Recipes, we'll see the Apple Pie Recipe no longer has a red line, so we can now delete this too. Just choose Actions > Delete, or select the tickbox and choose Delete from the dropdown at the bottom of the page. 