And you cannot will unless (it be) that Allah wills -- the Lord of all that exists.
This means that the will is not left to you all, so that whoever wishes to be guided, then he is guided, and whoever wishes to be astray, then he goes astray, rather, all of this is according to the will of Allah the Exalted, and He is the Lord of all that exists.
It is reported from Sulayman bin Musa that when this Ayah was revealed,
لِمَن شَأءَ مِنكُمْ أَن يَسْتَقِيمَ
(To whomsoever among you who wills to walk straight.) Abu Jahl said,
"The matter is up to us. If we wish, we will stand straight, and we do not wish, we will not stand straight."
So Allah revealed,
وَمَا تَشَأءُونَ إِلاَّ أَن يَشَأءَ اللَّهُ رَبُّ الْعَـلَمِينَ
(And you cannot will unless (it be) that Allah wills the Lord of the all that exists.)
This is the end of the Tafsir of Surah At-Takwir, and all praise and thanks are due to Allah.