The reason you have no error is because you are connecting only one PBRMetallicRoughness Block at the time to the Fragment Output.
Thing is, through the system of switches you want to implement, you would eventually need to connect all of these into some logic and at that point you would encounter the error referenced here (I tried to test out your approach and this is exactly the issue I’ve encountered :D)
As Evgeni_Popov mentioned, you need to move this switch logic before the PBRMetallicRoughness block, and end goal is to construct only ONE PBRMetallicRoughness material.