Why are you not allowed to overtake here now?

You selected the correct answer. Here, overtaking is not allowed because of the oncoming red car. According to German traffic rules, overtaking is only permitted when you can do so without endangering oncoming traffic. If a vehicle is approaching from the opposite direction and there is not enough space or time to safely pass, you must wait until the lane is clear. In this case, the presence of the red car means overtaking would be dangerous and is therefore prohibited. Always ensure the oncoming lane is completely free before attempting to overtake.