if you are currently in a correspondence audit with the IRS do not file anything unless you are specifically instructed to do so.
With respect to your 2020 return, you might want to apply for an extension if the matter is not resolved within the next four weeks or so.
Obviously, you should have received that 2018 K-1 much earlier than 12/2019, but the IRS is correct in the sense that the information on that K-1 should have been reported on your 2018 income tax return.
