Website not opening on a browser when www not entered

[DreamgiftStudio]

Hi , my store is at its final stages and i just found out that when i open my website on any browser without entering www then it gives and error but when i enter my website link as :" www.dreamgiftstudio.com" then it opens up as normal . any assistance would be greatly appreciated 

Also i donot know how to attach a screenshot along with .

the error window says :

Only one step left ! finish setting up your new web address , go to your domain settings , click "connect existing domain " and enter dreamgiftstudio.com  (which is my domain linked to my store )