When scheduling creatives within Campaign Manager 360, what's one thing that you need to do to make sure an appropriate ad will run every time?

Creating a default ad ensures that an ad will always be served, even if no other targeted ads are eligible for a given impression. This prevents blank impressions and maximizes ad delivery. Without a default ad, if a user's targeting criteria don't match any of your specific ads, no ad would be shown. Setting geographic targeting to a small number of regions or limiting to mobile devices would restrict ad delivery, not guarantee it. Creating an ad with multiple region-specific creatives is a targeting strategy, not a fallback mechanism for ensuring an ad always runs.