If you have purchased the domain from a 3rd party registrar (Google domains, GoDaddy, etc), make sure your domain is properly pointed to Shopify’s DNS records.
On your domain provider’s website, log in to your account.
Find the DNS settings or domain management area.
Change the following records:
Point the A record to the Shopify IP address 23.227.38.65.
If necessary, change the Host name to the @ symbol.
Delete any other A records on the domain if there are any present.
However, if your domain was purchased with Shopify directly, then you open a ticket with merchant support, they can look into it.
Contact page link: https://help.shopify.com/en/questions#/contact/