I feel this is a little unusual. Perhaps there is a small mistake, but the context might help to understand the exact meaning. Is it from something about witches or magic? The thing is that 'a charm' is an object like an an ornament or a piece of jewellery and often, but not always, people might believe it has some magical power (to bring good luck for example). Whereas 'charm' (without the 'a') is a quality of a person or thing. It's a quality of being attractive, inviting, friendly, well-mannered, etc.