Cannot create mailboxes due to missing Mail tab.
Mail server is not installed in Plesk:
Select the components you want to install:
1. [ ] Kaspersky antivirus
2. [ ] Parallels Premium antivirus
3. [ ] SpamAssassin support
4. [ ] Mailman mailing list manager support
5. [ ] Parallels Premium Outgoing Antispam extension
6. ( ) Postfix mailserver
7. ( ) Qmail mailserver
8. ( ) MSMTP relay only mailserver (SMTP client)
Install mail server through Plesk autoinstaller.
Apply the following article to resolve the issue completely.