Your state or local laws will dictate how long you have after you purchase a vehicle to get insurance on it. By contacting your state's department of motor vehicles or like agency you should be able to find out what your state laws are regarding this. Many states will want insurance coverage on a vehicle immediately if you are driving it on the roadways.

If the vehicle you were purchasing was replacing a vehicle on your existing insurance policy, it might be that your insurance company would have covered it at the moment in which you purchased it. It depends on the policy that you have and your insurance companies rules regarding this. If you find out that the truck was covered by your insurance company you might be able to get a document from them stating as this to take to the courthouse as proof of insurance and see if you can get your ticket dismissed.