WooCommerce - Adding SKU's to normal products

How do i add a SKU to simple product?

The product has no variations, it's just a one option product, but I can only add a SKU whenever I switch to a variant product or advanced product.

I need the SKU to show on the front end where it does on my variant product so if you customer wants to call in to order they can give me the SKU.

Hope someone can advise.

Thanks