Why cannot a website be accessed by url with leading www

I have hosted a website at one of digitalOcean's solutions and installed vestaCP on the server.

As I try to access the website without leading www (domain.com) I get no problems, but as I try to access it as www.domain.com, I just get an error that there is no such website.

I tried to research on that, but I really have no idea on how to get a grasp on the problem.