Frostbite is a spell from Xanathar's Guide to Everything (and, before Xanathar's was published, from the Elemental Evil Player's Companion ). If the creator of the game does not own Xanathar's Guide to Everything on Roll20, or has not enabled compendium sharing for that resource, the spell Frostbite will not be available to players. You can still edit your character sheet manually to include the spell, but without Xanathar's , it will not be possible to drag-and-drop from the compendium.
