If you believe you did not sign up for Marketplace but your return is being rejected, you will need to contact the Marketplace to discuss the matter. According to the IRS, "f you think you are not required to complete Form 8962 and include it with your return, you should contact your Marketplace to confirm that APTC was not paid to your health insurance company for you or any member of your family. After confirming that APTC was not paid for you or any member of your family, you should include with your return a PDF attachment titled "ACA Explanation" with a written explanation of the reason why you believe Form 8962 should not be required.
See the link below, on how to correct a rejected return based on a missing 1095-A.