Do you have any proof that this was supposed to be a 2019 contribution? Such as a notation on a canceled check, or a receipt from an electronic transfer? If you can prove that you intended it to be a 2019 contribution and the bank made the error, they should reverse it no matter when it happened.
If this was your mistake for not selecting the correct date pin an online form, then you're stuck. You need to amend your 2019 return to remove the contribution, and if you also made 2020 contributions, you will need to remove them so you don't have excess 2020 contributions.
