warlock level 5 spell slots
Scrolls - bg3.wiki
A level 1 spell fits into a level 1 spell slot, but a level 2 spell is too large for a level 1 slot – you can’t use lower-level spell slots to cast higher level spells.
Resources - bg3.wiki
Warlock spell slots Recharge: short rest (spell levels 1-5) Expended by warlocks to cast any spells they know. Warlock spell slots are all the same level, and any spells casted by expending them are upcast accordingly.
multiclassing spell slots - WebNovel
Warlock Spell Slots don't level up any higher when multiclassing, so if you want stronger Spell Slots you'll need to keep taking levels in Warlock.