I think many publishers only take submissions that have not been previously published, which include online. You'll have to check the guidelines from specific publishers. If you planned to self-publish with Amazon or something, then I wouldn't see any issue with it. Not sure Medium is the best venue to serialize fiction... I've heard of people using something called Vella, I think, although that may also be an Amazon affiliate. I don't know much about novel serialization so I'm not the best person to ask!