You enter an investment sale in TurboTax to account for the worthless stock. You list the proceeds of the sale as $0 and the cost equal to what you paid for the shares.
Follow these steps to enter the sale in TurboTax:
1. Navigate to Investment Income in the Wages and Income section of TurboTax
2. Choose the Stocks, Cryptocurrency, ... option
3. Skip the Import option
4. Choose the Stocks, Bonds, Mutual Funds option for investment type
5. Work through that section to find the Form 1099-B entry where you will enter the pertinent information to report the loss on sale of stock. You can enter "NA" for the name of the broker since you won't have one:

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