Do you know of any scholar who would have said that not praying Taraweeh (night prayers in the month of Ramadan) at all is a sin?

All perfect praise be to Allaah, The Lord of the Worlds. I testify that there is none worthy of worship except Allaah and that Muhammad, sallallaahu ‘alayhi wa sallam, is His Slave and Messenger.

Performing Taraaweeh prayer is an act of Sunnah and not an obligation. An-Nawawi cited the scholarly consensus in this regard and underlined that it is an act of the Sunnah. He wrote, "Taraaweeh prayer is an act of the Sunnah according to the consensus of scholars..." [Al-Majmoo‘]

Hence, the Muslim bears no sin for not performing the Taraaweeh prayer. However, he should not miss it in order not to miss its reward. Shaykh Ibn ‘Uthaymeen was once asked, "What is the religious ruling on the Taraaweeh prayer?" He answered, "It is an act of the Sunnah that should not be left."

We did not find any scholar who held that it is obligatory or that the one who does not pray it bears a sin for that.

Allaah knows best.