Third party sick pay
may or may not be taxable. If you got a W-2, with an amount in box 1, or a 1099-R, it is
taxable. The rule is if you paid the premium, with after tax money, the sick
pay benefit is tax free. If your employer paid the premium (as in most cases)
or you paid the premium with pre-tax money, the benefit is taxable.
I would appear the IRS got a copy of a W-2 or 1099-R and have added the taxable income. If you agree with them, just pay them. If you disagree, write to them and explain why.
If you want to verify the IRS numbers, enter your return and add the income. Do not save your changes, unless you made a copy or saved a pdf of the original return. If you decide to submit an amended return, see https://ttlc.intuit.com/questions/1894381-how-to-amend-change-or-correct-a-return-you-already-filed
