Lost in stock in the past but haven't claim

[PT12]

I filed a 2018 tax return but forgot to claim/report the lost in stock.  Can I still file for the 2018 lost in 2019 or 2020 tax return?  Thanks

[DoninGA]

An investment sale that was made in 2018 with either a gain or a loss has to reported on the 2018 tax return.  You will have to amend the 2018 tax return to report the investment loss.

See this TurboTax support FAQ for amending a 2018 tax return - https://ttlc.intuit.com/community/amending/help/how-do-i-amend-a-return-in-turbotax/01/27579