>>22025
If I wanted to say "You must not buy it / Buying is not allowed" it would be 買ってはいけない, which basically means "As for buying, it will not go well"
But "You must buy it" would use the negative form of to buy, 買わない (in the -te form) so it would be 買わなくてはいけない which basically is the same as the first one but saying "Not doing X is not allowed" or kinda directly translating to "As for to not-buy, it will not go well"

So, to answer your question, You must not do X-not is the closest approximation