Tafsir of Sad 38:27

{In vain (bāṭilan)}: A creation in vain, without a sound purpose or profound wisdom. Or, [it means] "as those who do vanity" (mubṭilīn), meaning playing/jesting, as in His saying: “And We did not create the heavens and the earth and what is between them in play. We did not create them except in truth” (al-Dukhān: 38-39).

The grammatical estimation is: "possessors of vanity" or "for the sake of jest." He placed bāṭilan (in vain) in the position of the verbal noun, just as they place hanīʾan (wholesomely) in the position of the verbal noun, while it is an adjective. That is: We did not create them and what is between them for jest and play, but for manifest truth.

That truth is: We created them as souls, deposited intellect and discernment within them, granted them capability, removed their excuses, then exposed them to great benefits through religious obligation (taklīf), and prepared for them an outcome and recompense according to their deeds.

{That}: This is a reference to creating them in vain.

{The assumption (ẓann)}: Meaning "the assumed thing." That is: creating them for jest rather than for wisdom is the assumption of those who disbelieved.

If you ask: Since they acknowledge that Allah is the Creator of the heavens, the earth, and what is between them—based on the evidence of His saying: “And if you ask them, ‘Who created the heavens and the earth?’ they will surely say, ‘Allah’” (Luqmān: 25)—then how are they considered to be assuming that He created them for jest rather than for wisdom?

I say: Because their denial of the Resurrection, the Reckoning, the Reward, and the Punishment leads to the conclusion that their creation is in vain and futile. Thus, they are treated as if they assume this and say it. For the recompense is the very thing for which the wisdom of creating the world was intended from the start. Whoever denies it has denied the wisdom from its root, and whoever denies the wisdom in the creation of the world has deemed the Creator foolish. It becomes apparent through this that he does not know Him nor does he esteem Him with the esteem He deserves; therefore, his acknowledgment of Him as Creator is as if it were no acknowledgment at all.

{Or shall We treat those who believe and do righteous deeds like the corrupters in the earth? Or shall We treat the righteous like the wicked?}