There was a lot of talk about this a few years ago. UEFA will come down hard on the host country if they just refuse to let a game happen. Like, VERY hard. But ultimately, UEFA is not the government; they have no way to force anyone to do anything.
But, long story short, it's probably a moot point - we may be playing behind closed doors:
https://inpaok.com/800260/uefa-pithano-to-kekleismenon-ton-thyron/