I'm filling out Form 3853 (Health Coverage Exemptions and Individual Shared Responsibility Penalty) and having trouble with the Exemption Certificate Number field. I'm trying to indicate that I don't have an ECN, but the form won't let me proceed. I've tried entering 'NO ECN' but it's not accepting this entry. What is the correct format or text I should enter in this field to indicate I don't have an ECN? This is for tax year 2024. Is this a system error?