Welcome.
If your car has the entry level Audio20 infotainment unit (as opposed to the higher end COMAND unit), then the navigation module is a separate option.
On early Audio20 NTG4.5 units, the optional navigation module is a Becker Map Pilot processor that fits into a bracket in the glove compartment.
On later Audio20 NTG5.x units, the optional navigation module is a Garmin Map Pilot SD-Card that fits into the SD-Card slot.
If you run the car's VIN in a free online VIN checker it will show which infotainment system is fitted to the car (and you'll be able to tell which navigation module you need to purchase).
Alternatively, you can opt for aftermarket Apple CarPlay / Android Auto adapter that will allow you to use the navigation software from your mobile phone.