Try reentering it — it must match exactly. If that doesn’t work, call 1-877-558-8352 for assistance.
The system may not recognize your VIN if your vehicle is a new purchase. Sometimes it takes a while for the VIN to appear in GM systems. If you have a new purchase and you are sure that you entered the VIN correctly — yet the system still does not recognize your VIN — register your vehicle by year, make and model.