Answer: 

This tab is not showing because the product is a Never Diminishing product and is not necessary, as physical stock of this product is not shown. 

Additional Information:

If you do need access to this tab and you are keeping physical inventory of this particular product, turn off the Never Diminishing setting in the product record. Ensure that any open sales orders or assembly transactions for this product are closed beforehand. 

How Can I Tell if a Product is Never-Diminishing?