there are three types of element to collect to build the base on the triceratops, wood, stone and rope, once you collect these materials, according to the amount that each one asks for, an indicator appears that informs you that you get on and you can now handle the triceratops, you press P on the white dino icon, and it is already enabled for you to handle it and you can complete the mission, the construction is only 70% of the game, so I invite you to do it try again and find out the ending ;)