Error buying realms

Me and my friend want to start a realm so we went on the website to go buy one but it says that I don't own the game. When I clearly do. To finish when we are able to buy it it says an error that says "This product is either not available for purchase, or you've found a glitch. Please try again later." 

 

Can someone explain to me how I could fix it because I really don't want to buy it again.