Stop creating activity on field service cloud?

Summary

How can I stop the external system creating activity on field service cloud that does not fit with respect to tech's availability?

Content

We have a use case where an external system should get an alert/intimation before creating activity on OFSC if the job does not fit on any of the tech schedules.

For example: If we select a slot of 5 PM - 6 PM and the duration of the activity is 1 hour. We have no tech on the field service side, who is working after 4:30 PM, so how can we get this type of information from field service (no tech is available).