The main reason you're likely not seeing "Flexible Ad Format" or "Dynamic Creative" (which Flexible Ad Format is slowly replacing) comes down to your campaign objective.
Here's what you can do,
Objective Matters Most: "Flexible Ad Format" is usually only available if you choose "Sales" or "App Promotion" as your campaign objective. If you're running "Traffic," "Leads," or "Engagement," it often won't show up.
Catalog Conflicts: Even with "Sales," if you're trying to use a product catalog (like for Advantage+ Catalog Ads), you'll need to toggle "Use a catalog" OFF in the ad set settings. Flexible Ad Format doesn't play nice with catalogs.
Conversion Location: For "Sales," make sure your conversion location is set specifically to "Website" (or "App"), not "Website and App" combined.
So, try creating a new test campaign with the "Sales" objective, and then check the ad set level to make sure "Use a catalog" is off and the conversion is "Website." Then, look for "Flexible Ad Format" at the ad level when you're building your creative.
If those specific settings don't bring the options back, then it might be a temporary bug and you'd need to poke Meta support again!
