ثُمَّ لَمْ تَكُن فِتْنَتُهُمْ إِلَّآ أَن قَالُوا۟ وَٱللَّهِ رَبِّنَا مَا كُنَّا مُشْرِكِينَ
Commentary:
The verse means that on the Day of Judgement all the mischief which disbelievers worked in their life on earth will end and nothing will be left to them save to deny their having ever attributed partners to God. This denial will really be a confession of their helplessness and a form of petition for Divine mercy.
This denial on the part of idolaters will really be a confession of their helplessness and a form of petition for Divine mercy.