The first thing we need to discuss is how would Order 66 fail? I think the only plausible way this could happen is if the inhibitor chips inside the clones malfunction. And as we've seen throughout the Clone Wars series it can happen so I think that is our best bet. As we see in Revenge of the Sith the Jedi and clones were fighting the separatists were becoming outnumbered by the Jedi and clones that is why the Emporer told the clones to execute order 66. So I think that the Jedi and the clones could defeat the separatist army and there would be peace across the galaxy. For about a day or two because as we have seen in The Disney plus show Star Wars the Bad Batch even those whose inhibitor chip hasn't affected them before can randomly make them do any think to execute the order until they have it removed. Now what do YOU think would happen if order 66 failed?