I checked out the domain mellonyofficialstore.com and it hasn't been registered with a domain provider, it is still available for purchase. If this is a typing error and you have actually purchased a different domain, please let me know.
If you'd like to purchase this domain directly from Shopify, you can do that by going to Online Store > Domains in your admin. You also have the option of purchasing it from a third-party domain provider, such as GoDaddy.
You can find more information on domains in this guide.
Please let me know if you are still having difficulty.
I took a look but your store seems to be working perfectly. If you're still having issues, please contact us here so that we can help you quickly.
I would love to give you some feedback on your store to help set up you up for success. If you're interested, please post on this board.