Why can you not turn off the road here?

Your answer is correct. You cannot turn off the road here because of the presence of the oncoming red car. According to German traffic rules, you are only allowed to turn or overtake when you can do so without creating danger or forcing oncoming traffic to brake or swerve. In this situation, since the red car is approaching from the opposite direction, turning or overtaking would endanger its driver and yourself. Always wait until the oncoming lane is completely clear before starting your maneuver. This ensures the safety of everyone on the road.