When the ACA was first implemented, you could be subject to a federal tax penalty if you weren't covered by a health plan that met or exceeded the ACA’s minimum essential coverage requirements. The federal tax penalty no longer applies. However, state penalties may still apply in some states. Check with a tax advisor in your state for more information.
If you are subject to a state penalty, different kinds of exemptions may apply to your situation: income-related, health coverage-related, group-related, hardship-related and more. If you believe you may be eligible for an exemption, you can learn more at Healthcare.gov about fees and exemptions.