Only the first one is correct

Basically you can say: Let's wait until ... Let's do it before ... If you are waiting, then you are not doing something. So, you can't say "let's not do it before ... " It's better to say, "Let's not do it." I hope it helps! :)

Hello Ei Shwe Zin! :) The first is correct. The second is impossible.

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Just the first