When adding a mailbox via command line the following error is being displayed (mailbox is still being added)
# plesk bin mail -c 'firstname.lastname@example.org' -passwd 'mypassword' -mbox_quota -1 -mailbox true
[2017-03-16 11:37:33] ERR [panel] Failed to connect to sitebuilder - missing or incorrect /usr/local/sb/include/init.php
SUCCESS: Creation of mailname 'email@example.com' complete
This error occurs if the number of actually created email addresses exceeds subscription limit for the number of mailboxes. Incorrect error display is recognized as product issue #PPPM-5921 and will be addressed in the future Plesk versions.
Check Resources of the subscription and ensure that the number of Mailboxes exceeds the number of existing email addresses + the number of email addresses you are planning to create.