Q. If someone didn’t hear the shofar during Elul in the morning should he blow at night? What if he has no minyan?

A. Poskim disagree whether one should blow the shofar at night Remoh (O.H. 581: 1) writes that there are places places where they blow shofar after maariv. Miley Deovos and Likutey Halevy maintain that this is to be understood literally referring actually to night proper (Nitey Gavriel – Rosh Hashana 4: 3: n. 7)
However some maintain that the Remoh refers only to before maariv when it is still day or after maariv if they daven early, since it is not an auspicious time for rachamim (Igrois Moishe O.H. 4: 21: 5. quoting Chaye Adam)
Aruch Hashulchan (581: 12) rules that one should not blow shofar at night since the shofar awakens the Heavenly Midos at an improper time. (See Piskey Teshuvos ibid. 3)
On question 354 on this forum we wrote: Whether Tekias Shofor during Elul requires a Minyan or no, would depend on the reason we blow. Tzitz Eliezer (12:48) writes that according to the motive of not forgetting the day when Moshe Rabeinu went up to Mount Sinai, this blowing may have been instituted only for a congregation. He also argues that even if the reason is the awakening before Rosh Hashono for performing Teshuvo, it may still require a Minyan, since a communal stirring is more effective than an individual one. However, Siach Yitzchok (264), Nitey Gavriel (Rosh Hashono 4:9) and others, rule that even a Yochid who davens on his own, if he has a Shoifor should blow.
Horav Shlomo Miller Shlit”a agrees with this last opinion, however, he should not blow at night.

Rabbi A. Bartfeld as revised By Horav Shlomo Miller Shlit”a.