If I enter only 18, it tells me to go back and enter 19.
Line 18 - Employee's EI premiumsTotal of box 18 from all T4 slips.Line 19 - Employer's EI premiumsEmployer's share of EI premiums (the employees' total premiums multiplied by the employer's premium rate).
Which slip are you entering on
assuming you are on T4, box 19 wont need to be entered, as it wont affect your tax refund, as it is the employeer's contributions