Yes, you can unlock all Skills in all skill trees in Horizon Forbidden West. However, to do so, you will need to earn enough Skill Points. These are earned by playing through Main Quests, Side Quests, Errands, and other side activities and objectives. By playing most of the game's Quests and other missions, you will gain enough Skill Points to unlock every Skill in Horizon Forbidden West, but it will take some time. For more information about Skill Points rewards, see our guide: Horizon Forbidden West: All Quests.