What if I already filed and then received an additional 1099-R that was not reported? How does the amended tax form work? Is there a penalty?

After your return has been fully processed you will need to amend to add the 1099R.   You will do this on a form 1040X, which is not yet available for 2024.   It should become available by late next week.

If you pay any additional amount due by 4/15, there is no penalty even if you amend after 4/15

@mspako1950