The cars do not have a physical car key, so you have to open and close the car with the app. The car will be available in the app 15 minutes before the scheduled pick-up time of your reservation. As soon as you are at the car, the app connects to the car and the orange Unlock the car button appears. If you tap on it, the car will open. Opening and closing the car works by means of Bluetooth. So, it is important that you have Bluetooth turned on and that the Amber app has access to use Bluetooth.
If you are standing next to the car and cannot get a connection with the app, it will probably help to force close the app and reopen it.