Allah tells them how much they wasted in their short lives in this world by failing to obey Allah and worship Him Alone. If they had been patient during their short stay in this world, they would have attained victory just like His pious close friends.
He will say: "What number of years did you stay on earth?"
means, how long did you stay in this world?
They will say: “We stayed a day or part of a day. Ask of those who keep account.”
meaning, those who keep the records.
He will say: “You stayed not but a little..."
meaning, it was only a short time, no matter how you look at it.
if you had only known! means, you would not have preferred the transient to the eternal, and treated yourself in this bad way, and earned the wrath of Allah in this short period. If you had patiently obeyed Allah and worshipped Him as the believers did, you would have attained victory just as they did.

Allah did not create His Servants in vain
Did you think that We had created you in play,
means, ‘did you think that you were created in vain, with no purpose, with nothing required of you and no wisdom on Our part?’ Or it was said that “in play” meant to play and amuse yourselves, like the animals were created, who have no reward or punishment. But you were created to worship Allah and carry out His commands.
and that you would not be brought back to Us?
means, that you would not be brought back to the Hereafter. This is like the Ayah:
Does man think that he will be left neglected? 75:36
So Exalted be Allah, the True King. means, sanctified be He above the idea that he should create anything in vain, for He is the True King Who is far above doing such a thing.