I am 6.4 inches and only21 is their any type of medicine or treatment that can help me increase in penis size?

None. There is nothing that will change size other that a terrible surgery full or risks and few results. Be confident in who you are. This is a confidence problem in your own mind, not in your partners mind i would bet. You are normal. Be happy with that. Be kind and confident and a good person. That counts, not size. .
Average. Your penis falls well within the average size range according to studies by trojan condoms. Other studies have shown that average size is preferred by female partners.