Story related, can't be missed
You will need to fill in both pages of your journal to activate the elevator. Start by interacting with all for tikis to fill in the first page, they are all situated around the huts on the lower level. Then interact with the signs above each hut to begin the second page. Roy's is being used as his headstone, so you will need to maneuver a little to get a view of it. The missing centerpiece is up the hill, in a hut, on the table on the right. Next to it is a jigsaw puzzle to complete to give you the last journal clue. Use these clues to input the tiki code into the nearby post.
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That gives you the missing piece from the elevator. Ride it up and go into the hole. Interact with the three eyes that whisper to fill in the next journal page. Go through the tunnel and hit the lever to open a shortcut to Harry's hut. On the table in the second hut is a device used to copy the eye pupil drawings in your journal.
Use the lense in the telescope in the corner to see a pattern. Copy that pattern on the floor in the hole with the eyes to complete the chapter and earn the achievement.