I think (hope) I’d be more helpful if I was at my computer and could do more copying and pasting from Epicor! But in the meantime…
Your SubQuery will work if you group by Company and HDCase.ProjectID as long as you then put MAX() around the DATEADD expression in the calculated field. My original approach assumed an additional containing SubQuery which did the same job, which I find users coming later can read more easily. But I think your error is because you haven’t grouped and aggregated in the same query.
I believe SubQuery criteria are a red herring unless I’ve misunderstood in some way. An aggregated SubQuery should do the job.