I have ISO stock I paid for several years ago that was cancelled (and I received no money for it) this year in connection with sale of old company. Where can I show loss?

You report that as an investment sale in TurboTax, with the sales proceeds listed as $0.

You enter investment sales in the Wages and Income section of TurboTax, then Investments Sales, then Stocks, cryptocurrency, Mutual Funds, Bonds, etc... Skip the section where it asks if you want to upload your tax documents. Choose Stock, Bonds, Mutual Funds as the type of investment you want to enter. You'll come to a screen where you can enter in your sales proceeds and cost basis.

Thank you, Thomas for your suggestion.  It is very helpful and I followed it. The program, however, still wanted me to attach a Form 8949 and I had to click in the Turbotax program the button stating that I would provide it to the IRS "offline" even though I do not have one to provide. Plus, the IRS apparently prohibits an individual from generating their own 8949 for federal tax purposes. I am going to put in a call to the IRS to see what they suggest for documentation. Wish me luck.