It’s not possible to delete a vehicle while a specific Automile Box is associated with that vehicle, to ensure all pertinent data remains with that vehicle in your account.
When you disconnect the unit from that vehicle and install it to another, you will have to follow the installation process again. You can find those instructions here
Once the unit has started to report data from the new vehicle, then you will be able delete the old vehicle from your list.
If you for some reason have more vehicles than devices or are having issues deleting vehicles please contact our support team via firstname.lastname@example.org or (415) 749-9270