Can owner cancel insurance if car is inoperable for 6 months and auto is at dealership?

It depends on:

• Is the car financed? If it is the finance company will not allow the insurance to be cancelled
• Is the car likely to be driven or moved by the dealership? if so they must be notified before the insurance is cancelled.

Other than that yes it can be cancelled.

Warning!Vehicles that are left on the dealership property can be sold as abandoned in as little as 90 days.

Is your car still there?