Thanks, Megan, I chatted with someone already in support. Because we ask for an attendee to upload a copy of their purchase order on the payment page, the system posts the error. I had to move the upload file question before the summary page in order to publish the web site.
Logically it's a little weird to ask for the upload of the PO before the payment page where you indicate that you are paying with a PO, especially when it is for a group.
Maybe uploading a PO can be made part of the payment module, so it won't interfere with the summary page anymore.