Inventory Option

I just recently purchased the download version of Desktop Quickbooks Pro 2021 for a custom product that we sell.  I have already set up the Inventory under the Inventory Part that we sell and had on hand at Dec 31, 2021.  After more research, I see that it should of actually been done under the assembly tab.  However, that option is not there.  The only options available is to add a Service, Inventory Part, Non-Inventory Part, Other Charge, Subtotal, Group, Discount and Payment.  How do I go about getting the Assembly option?