What is the reason for not being able to smell or eat foods that contain garlic or onions, even if a person could smell them and eat them without problems?

If this is a recent development one would have to suspect an infection by the Covid virus. If it is a longstanding feature it is a selective or incomplete altered sense of taste and smell which often has a psychological basis. If you still have concerns see an otolaryngologist at a university hospital who specializes in these disorders called chemosensory dysfunction.