This message pops up when a character (or all of them) is blocked on any part of the island. When this happens obviously they can't get to other parts of the island.

Please make sure that you didn't put any buildings or decor items on the path to the right side of the island or in front of the stairs on the left side of the island. Also check that you have not arranged the buildings and decor items in such a way that they surround the characters. If this is the case, you just need to move some of the objects to a more suitable place.