This week, Moshe Rabeinu in one of his last speeches to Am Yisrael refers to "this Mitzvah" and says that it's not far away, up in the sky or over the seas, but is in Am Yisrael's hearts to do.

What Mitzvah is Moshe Rabeinu talking about?

But further, when reading the psukim, it raises the contrast to when Am Yisrael got the Torah, and they said the famous "נעשה ונשמע", "We shall do and (only then) we shall listen", and it seems that here Moshe is describing the exact opposite situation - where Am Yisrael will complain they need someone to go bring it to them and explain it all and only then they will do it...

What happened to Am Yisrael? Why is Moshe describing such an opposite attitude??