﴿ يَوْمَ لَا يُغْنِى عَنْهُمْ كَيْدُهُمْ شَيْـًۭٔا وَلَا هُمْ يُنصَرُونَ ﴾

“the Day when none of their scheming will be of the least avail to them, and they will receive no succour.”

the day when their guile will avail them nothing yawma lā yughnī is a substitution for yawmahumu ‘that day of theirs’ and they will not be helped they will not be protected from the chastisement in the Hereafter.