you got x units on exercise some were sold to cover the taxes. thus the need to report the sale. from your company or broker get the basis. since the RSU's were sold to cover taxes the basis should be about the same as the sales price. get the basis from the company or perhaps it was shown on the 1099-B you should have gotten. the sale was reported to the IRS. you should send along an amended return reflecting the sale and cost when responding to the notice. include the 1040-x and related forms and schedules that are printed out for the amended return. make sure you sign and date it. mail using a method where you get proof of delivery. if you owe send a check for the balance due shown on the 1040-X.
FYI if you exercised RSU's in 2019 and didn't report the sale file an amended return to cut off the portability of getting a notice for that year.
