I am reporting selling stock I bought in 05/11/2017 and sold in 09/15/2020 (long term basis reported). I am getting, "cannot be before 01/01/2021.

[monomazzaro]

Why if I held the stock for > 3years?

[riskitall]

You sold your stock in 2020, which should have been filed for 2020, not 2021, that's why you are getting the error.

[DoninGA]

An investment sale in 2020 can only be reported on a 2020 tax return.  You will have to amend the 2020 tax return to report the sale

See this TurboTax support FAQ for amending a 2020 tax return - https://ttlc.intuit.com/community/amending/help/how-do-i-amend-my-return/00/27439