Komentarz do Powtórzonego Prawa 12:13

השמר לך TAKE HEED TO THYSELF [LEST THOU OFFER THY BURNT OFFERINGS IN EVERY PLACE THAT THOU SEEST] — This negative form of the positive command in v. 11, is stated in order to attach a negative command (לאו) to this matter (Sifrei Devarim 70:1).

בכל מקום אשר תראה, “in any place that you see.” This is a negative commandment; however, at the bidding of a legitimate prophet, under special circumstances, sacrifices may be offered on an altar outside the Temple such as Elijah’s at Mount Carmel when the purpose was to delegitimize the prophets of the Baal.

This places a negative commandment over the matter. Rashi is answering the question: Scripture had already written above (v. 11), “It is there you shall bring,” which implies only there and nowhere else. Why does it need to say again, “Take heed”? Rashi answers: “This places a negative commandment over the matter.”