...
Every staff member who requires calendar access must have a user record that is licenced to support Event object entries i.e. calendar entries. Other licence types can be used for non-calendar staff e.g. a general admin person could potentially use a Chatter licence.
A Chatter license will have this error when doing a Generate Attendance.
...
For more information about Salesforce licencing types, please refer to your PosiEd or Salesforce consultant.
...