Allah says:

﴿وَلَوْ شَآءَ اللَّهُ لَجَعَلَكُمْ﴾

(And had Allah willed, He would have made you) meaning - O mankind,

﴿أُمَّةً وَحِدَةً﴾

((all) one nation,) This is like the Ayah:

﴿وَلَوْ شَآءَ رَبُّكَ لآمَنَ مَن فِى الاٌّرْضِ كُلُّهُمْ جَمِيعًا﴾

(And had your Lord willed, all of those on earth would have believed together.) ﴿10:99﴾, meaning, He could have created harmony among them, and there would not be any differences, conflicts or hatred between them.

﴿وَلَوْ شَآءَ رَبُّكَ لَجَعَلَ النَّاسَ أُمَّةً وَاحِدَةً وَلاَ يَزَالُونَ مُخْتَلِفِينَ ﴾

﴿إِلاَّ مَن رَّحِمَ رَبُّكَ وَلِذلِكَ خَلَقَهُمْ﴾

(And if your Lord had so willed, He would surely, have made mankind one Ummah ﴿nation or community﴾, but they will not cease to disagree. Except him on whom your Lord has bestowed His mercy, and for that did He create them.) (11:118-119) Similarly, Allah says here:

﴿وَلـكِن يُضِلُّ مَن يَشَآءُ وَيَهْدِى مَن يَشَآءُ﴾

(but He allows whom He wills to stray and He guides whom He wills. ) Then on the Day of Resurrection, He will ask them all about their deeds, and will reward or punish them even equal to a scalish thread in the long slit of a date stone or the size of a speck on the back of a date stone, or even a thin membrane of the date stone.