Since you have already filed, you have two options:
1. Wait until the return has been processed (refund received or payment cleared), and then file an amended return to report the Form 1099-R or;
2. Wait until the IRS contacts you about the missing 1099-R, and pay the tax they propose in the notice they send. If the 1099-R is simple (e.g. doesn't involve an early distribution or have any other complicated characteristics), the IRS will propose an increase in your taxes and give you a payment coupon to use if you agree. This would be an automated notice and is very common, since many people either forget to report 1099-R income or never receive the form from the Payer.
Amending the return is the preferred course of action.
See How do I amend my 2020 TurboTax Online return?
