Short answer: undermixed batter, no skin before baking, or too much moisture about.
No feet means the batter couldn’t rise the way it should. If it was undermixed and stiff, it can’t lift into a frill — fold to a proper “flowing lava.” If you skipped the rest, there’s no skin to force the rise outward. And macarons hate humidity: on a damp day the skin takes longer to form, so give them more resting time. A low oven temperature can also leave them footless.
Related:raspberry French macarons · glossary:macaron feet,macaronage.