Being that you are using Turbo Tax Self employed, you either have a single member LLC or sole proprietor. There is no balance sheet on the tax return, so you would not necessarily need to enter inventory. However you do need the inputs to record the cost of goods sold. In the self employment section, after you have entered income, it gives you an opportunity to enter expenses. Scroll down to "less common expenses." You will see inventory there.
If you had no revenue, the purchases you made would not be expensed. You will have to wait until you have revenue. When you do have sales you would record it by entering the beginning inventory, the amount you spent on inventory during the year and the ending inventory.
