How do I deduct a long term capital loss from an unlisted security?

Yes, this transaction may be reported as an Investment Sale, found under Wages & Income >> Investment Income >> Stocks, Cryptocurrency, Mutual Funds, Bonds, Other.

The easiest way to find any section of TurboTax Online, is to open your return, go to Tax Tools in the left column >> Tools >> Topic Search. Type in your topic, click the topic in the list and then GO to go directly to the start of that section. In TurboTax for Desktop, use the Search box or Show Topics.

1. Click Start/Update or "Add Investments."
2. Skip import.
3. Choose Stocks, Bonds, Mutual Funds.
4. Enter a name to identify this sale. Nothing else is needed since you're not importing.
5. Answer the sale questions.
6. Choose "One by One."
7. Choose the appropriate sales section (long-term/short-term, basis not reported)
8. Type of Investment: Stock (non-employee)
9. Enter details of sale.
10. Complete the interview.