@danny_v1 yeah, PanelDue switch from M408 to M409 at some point, i.e. taking advantage of the object model available in RRF 3.x - so you'll need to match the PanelDue firmware version to the Sketch so they assume the same type of PanelDue communication.
You can upgade the PanelDue firmware, but you'll also need to run RRF firmware 3.x something, refer to the PanelDue release notes.