Your client needs to contact Healthcare.gov and see if any of the SSNs on their tax return has a 1095A issued to them,   

It happens ALL the time that people go online to see if they qualify and end up with a policy that they don't even realize they have, and they don't get canceled for non-payment. 

OR 

dependent child on the return is covered by another parent that has marketplace coverage and the 1095A from that other parent needs to be included on the return.

YOU cant fix it, your client needs to make some calls.