Phillips received 19.7% of the vote in the New Hampshire primary, despite Joe Biden not being on the ballot. Phillips received less than 2% of the vote in the South Carolina primary.

https://en.wikipedia.org/wiki/Dean_Phillips_2024_presidential_campaign

Each state sets its own primary rules, so they’re all over the place. If the guy can’t get enough support, he can still be a write-in.

Sorry, but that’s not preventing someone from running.