The mail tab is not present. It is not possible to set up mail accounts or activate webmail, etc.
Mail service is disabled on the server.
Mail service is not installed on the server.
Log into Plesk;
Check if the service is installed on the server. Go to Tools & Settings > Mail.
If it looks similar to this, mail service is installed:
Enable it by going to Tools & Settings > Mail > Mail Server Settings and enabling the option Enable mail management functions in Plesk:
If the the Mail section looks similar to this or is absent altogether:
Install mail components using the following KB article:How to install a mail server in Plesk