Best I can tell, this means the IRS thinks your client has marketplace insurance.  Doesn't necessarily mean the IRS is right but I don't think the IRS will accept it without an 8962.  I noticed Drake also has this reject code in their list of top 10 rejects this week so I don't think this is specifically a ProSeries problem.

Since our society is well on its way to Idiocracy, if someone on the return had a marketplace policy in the prior year, maybe it auto-renewed.  You might also look for ID theft.  Or, they certainly wouldn't be the first client who had marketplace insurance and didn't know it.

Rick