This error occurs if you don't have the relevant capabilities in the Scheduler activity. In order to see the "teacher side" of the activity, you need to have mod/scheduler:manage or mod/scheduler:attend; in order to see the "student side", you need mod/scheduler:appoint . See here for details:
https://docs.moodle.org/39/en/Scheduler_Module_capabilities
(In a standard setup, teachers, students and administrators will have the correct capabilities assigned automatically via their roles, but it may be that you are using customized roles which would need to be updated manually.)
https://docs.moodle.org/39/en/Scheduler_Module_capabilities
(In a standard setup, teachers, students and administrators will have the correct capabilities assigned automatically via their roles, but it may be that you are using customized roles which would need to be updated manually.)