Tafsir of Az-Zumar 39:4

لو أراد الله أن يتخذ ولدا ﻻصطفى مما يخلق ما يشاء

Had Allah willed to take a son (or offspring), He could have chosen whom He willed out of those whom He created. meaning, the matter would not have been as they claim. This is a conditional sentence which does not imply that this happened or that it is permitted; indeed, it is impossible. The aim is only to point out the ignorance of their claims. It is like the Ayat:

لو أردنا أن نتخذ لهوا ﻻتخذنه من لدنا إن كنا فعلين

Had We intended to take a pastime, We could surely have taken it from Us, if We were going to do (that). (21:17)

قل إن كان للرحمن ولد فٲنا أول العبدين

Say: "If the Most Gracious had a son, then I am the first of worshippers.'' (43:81) All of these Ayat are conditional, and it is permissible to form a conditional sentence referring to something that is impossible if it serves the purposes of the speaker.

سبحانه هو الله الواحد القهار

But glory be to Him! He is Allah, the One, the Irresistible. means, exalted and sanctified be He above the idea that He could have any offspring, for He is the One, the Only, the Unique, the Self-Sufficient Master to Whom everything is enslaved and is in need of. He is the One Who is independent of all else, Who has subjugated all things and they submit humbly to Him. Blessed and exalted be He far above what the wrongdoers and deniers say