I was able to register my car, so how could this tax still be due?

Although state law prohibits anyone from registering a vehicle with outstanding motor vehicle taxes due, the Registry of  Motor Vehicles is not able to identify and match all persons owing such taxes.  It is not unusual for persons owing motor vehicle taxes to be able to register a vehicle.  The fact that you were able to register a vehicle with this tax being due is not proof of payment of these taxes.

All motor vehicles registered in your name will be taxed each year by the city or town listed by you with the Registry of Motor Vehicles at which the vehicle will be kept.  This tax is referred to as the Motor Vehicle Excise Tax and that tax is assessed in accordance with Title 44, Chapter 34 of the General Laws of the State of Rhode Island.