If its a manual box you have to have it in neutral, mine wont go in until i take it out of gear, the lights on the dash will just flash and keep on flashing until i take it out of gear, then it will go "click" and drop straight in.
If its an auto then you will have to have your foot on the brake, and maybe have it in neutral or park but im not 100% sure about the last bit, im sure someone will confirm this, Old Tony has an auto pm him and he will soon give you the run down.