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.