When you open the vehicle with your badge, the terminal in the glove box asks you for a 4-digit PIN code according to the dd/yy scheme (example: if you were born on 24/06/1995, your PIN code will be 2495).
When creating your account, make sure you have not made a typing error in your date of birth.
If your PIN does not work, a message in the vehicle may tell you that your reservation is complete, and that the doors will lock in 5 minutes. Simply exit the vehicle, lock the doors with your badge, then pass the badge again to unlock the doors. This will restart your reservation at 0 and you will be able to enter your 4-digit PIN again, composed of your day and year of birth.