Exactly. Wouldn't you translate "You may not drive" as "Je mag niet rijden" as you might not be driving a car, it could be a lorry or motorcycle.
I suppose there is a slight problem as "Je mag niet rijden" could translate as "You may not ride [a horse]", so using "drive" shows that we have spotted that is a car.
But I don't think it is wrong in English to put all the information about what you are driving.
In Dutch, 'Je mag niet' is a very definite order that must be followed. English 'may not' was once also a definite no in 'the old days', but has become more relaxed over the years. If your father said that "you may not leave the house today" that was understood as an order. And if a child asked "Can I go out and play?" the adult would answer "You can, but you may not.", and you would have to re-phrase your question and ask "May I go out and play?" Language has changed a lot over the years.
I wonder if the problem with your translation is with "can't".. which would translate to "kan niet" versus "may not" which is "mag niet"... but i am not a native dutch speaker so i am not sure. i left out other words for brevity, so i hope you understand what i am trying to say.
I had 'You mustn't drive' marked wrong. I've spoken English in GB all my life and I think that 'You may not drive' would hardly ever be said - it would sound incredibly stilted and unidiomatic. I've noticed that Duolingo has a built-in dislike to ever using 'must'. It's always 'have to', or 'not allowed to' in the negative. Is it that Americans never use the word 'must'? In Britain we use it all the time, and there is a difference: 'have to' and 'not allowed to' tend to suggest an obligation imposed externally, whereas 'must' is an obligation that tends to come from oneself.