Assassin's Creed Valhalla

Abilities are unlocked by obtaining Books of Knowledge (marked on the map as yellow dots, tracked as wealth). When you get near enough to the yellow dots, they will turn to books with a rune on the cover. You'll need to collect 8 different ones first (there are over double that many in the game), at least 4 melee ones and at least 4 ranged ones, and then go to the Abilities tab from the pause menu. Here, map 4 melee ones to the four available slots, and do the same with 4 ranged abilities. Once all 8 slots are filled with abilites, this achievement will unlock.
 
Note that you'll need to collect all Books of Knowledge in the game for the Completionist All the Way! (30G) achievement. Refer to it for locations of all Books.