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

Why are you trying to enter a 2019 sale on a 2020 return? Online is only for 2020 returns. Do you need to prepare a 2019 return or amend it to add the 1099B?

short sales opened in 2019 and closed in 2020 would be reported on 2020 return. in this case, the date of the sale would show as in 2019 but the purchase date would be in 2020