I read that しなくてはダメ means “to have to do something” but someone said ダメ cannot be used with なくては to mean “have to”. I get different answers from different sources, both being Japanese speakers. What is correct?
しなくちゃダメ、しないとダメ、しなくてはダメ are all used very frequently in Japanese, as a simple Google search (with quotation marks) will quickly demonstrate. 駄目 itself is often considered somewhat informal (compared to なりません, いけません etc）though, so maybe that's what your other Japanese source was referring to.