Finish The Required Mission
The most important thing you have to do before you get started on Far Harbor is complete a prerequisite mission. For those of you who have poured tons of hours into Fallout 4, this shouldn’t be too much of a problem as the mission is one that you come across pretty early on in the main quest of the game.
The quest, Getting a Clue, is one that you’ll complete with Nick Valentine at your side. With Valentine being an integral part of your adventure in Far Harbor, it makes sense that you’d have to have all other story elements finished with him before you can continue. If you’ve just picked up the game or have just hardly touched the main quest line at all, now would be a good time to start completing missions so you’re all ready for Far Harbor.