I had a similar problem when applying for marketplace insurance at the end of 2020 (for the 2021 year). Clearly, a child that is not a tax dependent can be included in a family health insurance plan BUT the marketplace system wouldn't allow me to do that. I spoke with 3 or 4 different people and pointed them to examples in the regulations and the tax forms but I couldn't get them to budge. They kept insisting that the tax non-dependent would need her own policy. I believe it might have worked if I checked the box to entirely forego the advanced payments of the PTC but I didn't want to do that. In the end, I re-arranged my affairs to pay more than 50% of her total support to make her a tax dependent and then I included her on the marketplace application as a dependent.
I've been told that the marketplace application seems to have a similar glitch with children that might qualify for CHIP or medicaid. Such a child should be able to be included in the family insurance plan (although the child would not qualify for PTC because the child qualifies for minimum essential coverage). I understand that the marketplace application doesn't seem to allow for such children to be included on the family insurance plan.
