To start the Audi, simply press the START/STOP button to the right of the steering wheel. For the vans and the Fiat, there is an ignition key located in the glove compartment on the right. The most common cause of error at the start is that you are not pressing the brake firmly enough. Just give it your all and press it as hard as you can ;).
Another reason is the steering wheel lock, especially on vehicles that are started with keys. Turn the key while moving the steering wheel from left to right. If it still doesn't work, our service hotline will be happy to help you. +49 (0) 30 / 2016 4975.