Most likely, it means your vessel was manufactured or imported before 1972. Those vessels are exempt from having a HIN/Hull Identification Number.
It could also be that your vessel was homemade and not from a manufacturer. Should that be the case, apply either to your State DMV or through us for Coast Guard Documentation.
If your vessel measures more than five net tons and is wholly owned by an American citizen, your vessel is eligible to be documented. File for vessel documentation here.