How can I write-off or deduct a worthless C-Corp preferred stock investment made in 2016? A private (not publicly traded) technology venture that is now not operating.

Yes. You enter your worthless stock as an investment sale in TurboTax as follows:

1. Choose the Federal option on your left menu bar
2. Choose Wages & Income
3. Choose the Investments and Savings (1099-B, 1099-INT, 1099-DIV, 1099-K, Crypto) option
4. Choose Add investments
5. Choose the Stocks, Bonds, Mutual Funds option and follow the instructions

Indicate that you don't have a form 1099-B. Enter the sales proceeds as $0 and the cost basis as what you paid for the stock. The date sold is the date it became worthless. You should report the sale in the year the stock became worthless.