You need to remove the W2 completely from your return. It sounds like you may have still entered the W2 on your return and the IRS will not allow a W2 with a zero in box 1 to be e-filed.
Return to your W2 section under Federal Taxes -> Wages and Income and make sure to completely delete the W2 associated with the third party sick pay. Also, make sure that there are not any blank copies if you started it and then stopped when found out don't have to include it.
