There's no reason why you can't kiss a girl during her period. Kissing is a normal and intimate act that has nothing to do with a woman's menstrual cycle.
You might be wondering about this because of cultural or societal myths surrounding menstruation. However, there's no medical or scientific evidence to suggest that kissing during a woman's period is harmful or undesirable. In fact, many people find intimacy and affection comforting during this time, and a kiss can be a nice gesture of support.
If you're unsure about how to approach this, it's always best to communicate openly with your partner. Ask if there's anything you can do to make her feel more comfortable during her period.
Ultimately, whether or not you kiss someone is a personal decision based on your own relationship dynamics and comfort levels.