How to put a "Buy Now" button in blog

I have a WordPress.com site where I paid for the domain and have a .com and am now trying to put a buy now button for something I’m trying to sell. Though, it doesn’t let me do it with the Paypal code. I am using the free version of WordPress. Do I need to pay and upgrade to insert a “buy now” button? As well, why can’t I do any of the plug ins? It seems like a basic feature, but I don’t have it in my sidebar of options

Thank you very much!