Hyde here from Shopify.
Did you buy your domain from Shopify, or is it a third-party domain? If you bought it through Shopify, you could try clicking "connect existing domain" and submitting the name of the domain that you bought from Shopify. The more context and information you can give me, the better I can help you. In the meantime, here are all our guides on using domains in your store!
I look forward to hearing from you!
All the best, Hyde.