The way SKUs work is the following:
1. All products without variants can have an SKU
2. When you product has variants, then:
2.1. If the variants are automatically created, the SKU is disabled (as having multiple variants using the same SKU does not make much sense).
2.2. If the variants are manually created, then you can add an SKU for each of the variants by clicking on "Add a SKU (Stock Keeping Unit) for the variants" in the "Edit products variants" screen.
To use SKUs on products that have variants you just need to use the option to manually create the variants.
> I would like to add SKU on my products but the site is not giving me Inventory options