Is it permissible for a Muslim to rebuke his brother for abandoning a recommended act?

There is no denunciation regarding recommended matters. Rather, what is recommended in such cases is advice, guidance, and encouragement, as the Prophet, peace and blessings be upon him, did when he said: "Whoever stands in prayer during Ramadan with faith and expecting Allah's reward will have his past sins forgiven," and "Whoever performs ablution like this ablution of mine, then prays two rak'ahs in which he does not speak to himself, will have his past sins forgiven."

It is well-known among scholars that they say: "There is no denunciation in matters of disagreement," and this is generally true, unless it contradicts a text from the Qur'an or the Sunnah or a consensus. Therefore, there is no denunciation for leaving a recommended act, nor for a matter of disagreement unless it clashes with a text.