Before starting up Fallout 4’s next DLC make sure that the mission “Getting a Clue” is completed. “Getting a Clue” is the main storyline quest that follows right after the “Unlikely Valentine” quest and is necessary in accessing the Far Harbor DLC. “Getting a Clue” starts in Diamond City in Nick Valentine’s office and involves the sole survivor teaming up with him and Dogmeat to look for clues surrounding Kellogg. The quest ends and leads into the next quest, “Reunions”.
Since Far Harbor involves the player teaming up with Nick Valentine, it makes sense that the quest is required to access the DLC. However, this quest is completed fairly early in most playthroughs so it should not be a problem for most players. However, if this mission is not completed for you, there is still some time to complete it before the DLC comes out. Which is probably why the official Fallout Twitter account let players know about the requirement before the DLC dropped.
Far Harbor will be available May 19. Watch the trailer here.