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
