Do I have to have a bar code in order to sell my product in a store?

Usually yes, most stores where inventory from many companies is combined do require globally unique bar codes, aka UPC/EAN "GTIN's". This is because if stores were allowed to create their own identifiers they would eventually conflict and there would be payment & inventory issues.

The only exceptions are for very small "mom & pop" stores that do not have Point Of Sales systems and manually track sales, or if you yourself are the store (you own your own store or website) and you're selling your own products directly to the customer.