After you purchase a domain, its registration can require some time. The reason for the delay is that name servers update their zone information periodically. Thus, until all name servers have received 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 Preview
To view preview of your websites:
1) Login in to your Plesk Control Panel.

2) Click "Subscriptions" at the left.

3) Click on subscription name.

4) Click "Websites and Domains" tab.

5) Click on Preview option.

6) Here you can see the preview of your website in red marked box.
