In the business info section you must indicate you have an inventory and then the COGS section will appear in the expenses section of the Sch C input area.
Here's some information I provide, because 99% of the time, first time users of the COGS section get it wrong.
Beginning of Year (BOY) Inventory - This is what *you* paid for the inventory in your physical possession on Jan 1 of the tax year. It flat out does not matter in what year they were purchased. If this is your first year using COGS, then your BOY Inventory *must* with no exceptions, be ZERO. This is because your BOY inventory must match the previous year's EOY inventory. If this is your first year using COGS, then your EOY inventory last year was zero, without question.
End of Year (EOY) Inventory - What *you* paid for the inventory in your physical possession on Dec 31 of the tax year. It does not matter in what year you paid for that inventory either.
Cost of Goods Sold (COGS) - What *you* paid for the inventory you actually sold during the tax year. It does not matter in what year you paid for that inventory either.
Note that you can only deduct from your taxable business income, the cost of the inventory you actually sold during the tax year. Again, it doesn't matter in what year you purchased that inventory either.