After you purchase a domain, its registration can require some time. The reason of the delay is that name servers update their zone information periodically. Thus, until all name servers get information about your zone, you will not be able to access your site by its domain name. Nevertheless, you can always access your site during domain name propagation using the button on the Websites & Domains tab.
During domain name propagation other people can access your site too. For this purpose, Panel registers your website as a subdomain of one of provider's sites. For example, if your website is my-domain.tld and a provider has configured the preview service on provider-domain.tld, you can access your site on my-domain.tld.192-0-2-12.provider-domain.tld. Here 192-0-2-12 is the site's IP address where dots are replaced with dashes.
Note: If you do not have the site preview button on the Websites & Domains page, contact your hosting provider.