Invariants have to be declared with a base of the context node on which the xpaths are to be evaluated. And in the case of these xpaths, that's MedicationStatement, not MedicationStatement.reasonNotGiven or MedicationStatement.reasonForUse[x]. Also, you can't do an exists on reasonForUse[x]. You need to say exists(*[starts-with(local-name(.), 'reasonForUse')])