In Any Light by Sam Burns

Isla MacKenzie is probably the strongest witch alive who can’t do magic. Her mentor died and left her with not one, but two jobs that would be difficult for any functional witch to fulfill, and she can’t manage so much as a simple cantrip to get started.
Add to that her growing feelings for Cassidy Simon, the town’s vampire bartender, and she feels like she’s on the precipice of something huge . . . she’s just not sure if it’s going to make her stronger, or kill her.