@joanne19 , since the check was issued in 2019 ( they must have issued informational return to the IRS -- generally a 1099 ), and IRS was asked you about it -- suggest you accept IRS notice -- there is a portion there allowing you to agree and pay the amounts due --- also send along a note asking for forgiveness of penalty -- they may waive that.
For 2020, if your return has already been processed, please file an amended return to cover the income received for 2020 -- IRS will eventually catch up and will ask for interest and penalty. Note that once your return has been flagged for missing income ( non-compliance ) , you need to make sure that for the next couple of years you are meticulous about recognizing all incomes.
Good Luck
