Where do you enter a investment stock loss? Can you enter it in Turbotax self employed?

Yes, you can enter a stock loss in TurboTax Self-Employed. But, you need to report the stock loss in the year that it occurred. From your comment, it seems the stock loss may have happened in a prior tax year before 2021. If that is the case, you can amend the tax return from the year of the loss. You will be able to carry forward the loss to later tax returns.

If the loss on the stock did occur in 2021, you can enter this in the investment section. You will need to determine the cost basis for the old investment. Some websites have information about historical stock prices. Make an educated guess about the price paid and the date acquired for the stock sale. As Hal_Al mentioned, the stock may be worthless or might have sold for very little.

To enter an investment sale other than from a brokerage account (1099-B), follow the steps below in TurboTax Desktop

• Click on Federal Taxes 
• Click on Wages and Income 
• Click on I'll choose what I work on 
• Under Investment Income
• On Stocks, Mutual Funds, Bonds, Other, click the start button

Or in TurboTax Online, enter investment sales in the Search box located in the upper right of the program screen. Click on Jump to investment sales

1.  Did you sell stocks, mutual funds, bonds, or other investments in 2020? Click on Yes
2. OK, what type of investments did you sell? click on Other and then on Continue
3. Continue past the screen Tell us more about this sale
4. On the screen Now we’ll walk you through entering your sale details, select the type of investment sold in the dropdown on What type of investment did you sell?
5. Continue with this section until it is completed.

@Hal_Al