You mean for free file forms you don't like getting an email with some incomprehensible language? Oh, don't worry they provide a tool. Just copy the _entire_ email they send you and paste in some tool.
Genuinely it is a horrible experience, but I suppose it is better filing by paper and waiting a few weeks for a small correction.
Well, I thought so too. Until I realized six months later that I hadn't gotten my refund. Turns out that I'd gotten an email shortly after submission that had gone unnoticed saying that my return had failed xml validation, just like you. What was the point of the frontend validation before submission? Why isn't the full validation done before submission properly? Total nonsense.
I re-filed with paper... still hasn't showed up on the IRS website.
Genuinely it is a horrible experience, but I suppose it is better filing by paper and waiting a few weeks for a small correction.