I purchased stock 20 years ago. Unbeknownst to me, the company went bankrupt / declared worthless in Nov. 2015. I just discovered. How to claim for the capital loss?

Since the stock became worthless in 2015, you have to claim the loss on your 2015 tax return. You have to file an amended return for 2015 to claim the loss. Report it as if you sold it on December 31, 2015 for $0.

Normally an amended return has to be filed within 3 years of the due date of the original return. But for worthless stock you have 7 years, so you have until April 15, 2023 to file the amended return (but do it as soon as you reasonably can). You will need a copy of your 2015 tax return in order to prepare the amended return.

Keep whatever documents you have showing that the stock was declared worthless in 2015, and also documentation showing that it was not worthless at the end of 2014. Keep these documents for 3 years after you file the amended return.

You will not be able to use TurboTax to prepare the amended return because TurboTax for 2015 is no longer supported or available.