﴿ وَمَآ أَرْسَلْنَٰكَ إِلَّا كَآفَّةًۭ لِّلنَّاسِ بَشِيرًۭا وَنَذِيرًۭا وَلَٰكِنَّ أَكْثَرَ ٱلنَّاسِ لَا يَعْلَمُونَ ﴾

“NOW [as for thee, O Muhammad,] We have not sent thee otherwise than to mankind at large, to be a herald of glad tidings and a warner; but most people do not understand [this],”

And We did not send you except to all kāffatan is a circumstantial qualifier referring to al-nās ‘mankind’ and is made to precede li’l-nās because of the importance attached to it of mankind both as a bearer of good tidings to believers of Paradise and a warner to the disbelievers of chastisement but most people such as the disbelievers of Mecca do not know this.