What is the ruling on the prayer of one whose Imam made a mistake in the Tarawih prayer, so he said "Allahu Akbar" and stood up after the second prostration of the second rak'ah, then sat down and completed the tashahhud and saluted without performing the prostration of forgetfulness, claiming its illegitimacy in supererogatory prayers?

Prostration of forgetfulness (sujood as-sahw) is required in both supererogatory (nafl) and obligatory (fard) prayers. The Imam was supposed to perform two prostrations after the salam, and the congregants were also supposed to perform them if the Imam did not. The prayer is not invalidated by omitting the prostration of forgetfulness, because it is a补 (jābir) for the act of worship.