Any investment sale occurring in 2019 has to be reported on a 2019 tax return, Not on a 2020 tax return.
You will need to amend the 2019 tax return that was filed to add the investment sale to the 2019 tax return that was filed.
See this TurboTax support FAQ for amending a 2019 tax return originally completed and filed using TurboTax -
https://ttlc.intuit.com/community/amending/help/how-do-i-amend-my-turbotax-online-return/01/27577
