You do realize the creator of this mod is deceased and has sadly passed away right?
On to the topic of spawning vehicles. You can spawn vehicles if you have
Juiced installed, press Insert and type
spawn_car VEHICLENAME VARIANTNAME
Vehicle name being a actual vehicle definition, such as van_2dr_riot01 being the peacekeeper. And variant being for example ultor, which will then spawn the ultor version of the peacekeeper. If you don't care about the variant, you can just leave the variant name blank.
You can make it so sandbox and or sandbox++ can spawn vehicles but it's very limited and not recommended as you have to create groups for the spawnable vehicles, then you kinda have to also call some lua command to change the variant and it's kind of blah and jank at best and that's explaining it lightly. Your best course of action is used juiced and use its native spawn car command.