On Rosh Chodesh, as well as any time that Tachnun is not said...

Nshei Chabad Newsletter / On Rosh Chodesh, as well as any time that Tachnun is not said, such as Maariv every day or Mincha on Erev Shabbos, one does not beat on the chest while saying the B'rocho S'lach Lonu in Sh'mone Esrai (Likutei Dinei Uminhogei Rosh Chodesh , Ch. 7, Par. 5).