You will need to reject the car I’m afraid. If you have yet to take delivery, don’t accept it. If you have taken delivery it’s a bit more complicated as, by entering the PIN, you have agreed that the car is what you ordered and that you are happy with it. However, in this instance, I have no doubt that Motability will allow you to order another car that is suitable for a towbar. Give them a call and explain. They are usually very helpful and understanding.