We're using Moodle 2.1+, so not sure which version you're referring to, but when you installed the plugin it should have added a manager's email address option to the profile field that needs to be completed.
The other option is to turn the managers' notices options off for the particular face-to-face that you're having trouble with. That should stop it from preventing the system to work.
Another global option (Site Administration>Plugins>Activity modules>Face-to-face) that you can turn on is to let the students add their own managers.