This request will come from Small Bertha in Far Harbor, as part of the main quest “The Changing Tide”. This requires clearing the Echo Lake Lumber Mill. The area is overrun by feral ghouls so you just need to explore the area and kill all ghouls to clear it.
Once done, report back to Small Bertha. She’ll lead you to the captain so follow her and talk to Bertha again. The doctor will be amazed with your help and give you the exp reward and complete the quest. You can now use the Lumber Mill as your new settlement in the island.