You attached a PDF and when you chose the "type" of attachment you chose ACA Explanation in the drop down menu?

FWIW, make sure your client has called the marketplace and made sure that none of the SSNs listed on the return has a policy, sometimes it happens by accident, sometimes its some kind of ID Theft, the attached ACA explanation will get it accepted electronically, but they'll still need to deal with the IRS letter that follows, any refund will be delayed, give them fair warning.