Connecting a domain to hosting in Plesk requires consistent configuration of both the hosting subscription and the DNS records. Both need to point to the right service.
Check the subscription in Plesk
After logging into the panel, make sure the domain has been added to the correct subscription. That subscription is where the site's files, databases, SSL, and email all run.
Set up DNS and wait for propagation
Point the domain to the correct DNS servers, or set the A, CNAME, and MX records. Changes may become visible after a few minutes, or after a few hours.
Check the website and SSL
Once propagation is complete, check the site with and without www, the SSL certificate, any forms, and email at the domain.