It is possible to have a schedule with gaps where no one is on-call during certain times. If no one is on-call at a certain escalation level, the incident will immediately escalate to the next level of the escalation policy. If no one is on-call for the entire escalation policy, an incident will not be assigned.
If you try to create a new incident in the Web UI on a service where no one is on-call, you will need to specify who it should be assigned to.
When services automatically try to create incidents, they will be created, but remain unassigned.