The navigation function cannot be selected when the GPS module is not connected to the car stereo. To resolve this, make sure that the GPS module is properly connected to the USB port of the main unit. Then, tap Navigation again. If the issue is still not resolved, service may be required. Go to Product Repair.
NOTE: Refer to the manual supplied with the car stereo for model-specific information regarding the model of the GPS module required and how to connect the GPS module.