The 2020 Form 1099-R can only be entered on a 2020 tax return. If you filed a 2020 tax return then you will have to amend the 2020 tax return that was filed to add the 1099-R.
See this TurboTax support FAQ for amending a 2020 tax return originally completed and filed using TurboTax - https://ttlc.intuit.com/community/amending/help/how-do-i-amend-my-return/00/27439
