Your spellcasters have a limited number of remembered spells they can use in Baldur’s Gate 3. As useful as it would be for them to readily use every spell and ability at their capacity, specific spellcasters, namely wizards, can only use a certain number during combat. Your party needs to be at a full stop and not doing anything aggressive for a wizard to take a moment to prepare their spells.
To prepare any of your party’s spells, make sure you out of combat, and nothing is actively happening. Your party can be in the middle of the wilderness, in a friend camp, or anywhere safe. They can even be in a safe location in the middle of hostile territory. The important thing is to do all of this out of combat.
Once you’ve double-checked this, open your spellbook to view your party members’ actions, cantrips, and various spells. The prepared spells that your spellcaster can use are the highlighted ones, whereas the ones they do not have prepared and cannot use in combat are darker than the others. To exchange those out, click on the ones you don’t want to use to unhighlight them, and then pick the one you want to use. These spells have to be at the same level. You cannot unprepare a level 1 spell, and instead, use a level 2.
After you’ve finished picking your new options, doublecheck they’re available on your hotbar at the bottom of your screen to readily use them in combat. You can make the hotbar bigger for any party member by clicking the small plus sign next to their spells, actions, and items.