Why are you not allowed to take the turn now?

You got it right. In this situation, you are not allowed to take the turn because you are in a lane marked with a mandatory direction arrow, which means you must follow the direction indicated by the arrow. In Germany, if there is a road marking with a straight or turning arrow in your lane, you must obey it, even if the traffic light is green and you have right of way. Ignoring such lane markings is a traffic violation. Always choose your lane early and make sure you follow the mandatory direction to avoid mistakes like this in future situations.