https://www.pcgamer.com/new-players-cannot-currently-join-sea-of-thieves/

The open world multiplayer pirate-'em-up Sea of Thieves launched yesterday, and in the finest tradition of online game releases, it was a mess. Our own hearty band of brigands struggled to get aboard until late last night (although they still managed to put together this handy beginner's guide), and developer Rare acknowledged on Twitter today that a large number of players are also having issues. 

The situation is apparently proving tougher to resolve than expected, as Rare has now put a temporary halt to new players joining the game.