Which sentence is correct?
- Event is raised when maintenance occurs or scheduled.
- Event is raised when maintenance occurs or is scheduled.
Different verbs have different transitivity, which means that some can take no direct object, some one, some more, and some are flexible in this regard.
Occur is an intransitive verb, and takes no object, so we can say "maintenance occurs".
Schedule is a transitive verb, that must take a direct object. So we cannot say "maintenance schedules" here as that means that the maintenance is what does the scheduling. You can though say "the engineer schedules maintenance". ("maintenance schedules" also leaves us wondering what it schedules. Even "The engineer schedules" is a stretch of the word, as it really should have an object).
We can though use a transitive verb in the passive voice, so the subject becomes the thing acted upon rather than the thing acting: "maintenance is scheduled".