(إِنَّ مَا تُوعَدُونَ لَآتٍ ) (Indeed, what you are promised is coming): When a promise is made by The Powerful; Who, nothing can ever cause him failure, neither in heavens nor on earth, then we can be sure that His (swt) promise will be inevitably fulfilled. Everything that Allah (swt) promises; Heaven or Hell, accountability, declaration, or standing between His (swt) hands on the Day of Judgement, will surely be fulfilled.

(وَمَا أَنتُم بِمُعْجِزِينَ) (and you will not cause failure [to Allah]): Never will Allah (swt) be caused failure, nor can anyone leave the realm of His Authority (swt).