A subform control can only contain one form
Older versions of Access allowed 3 levels of nesting but newer versions allow 7 (I think)
Of course only a subform in single view can have a subform regardless of Access version. So if your form/subform is continuous or DataSheet, It cannot have any subforms at all. That would force Access to render the subform for each visible instance of the continuous or DS subform so that is what causes the restriction.