"And they say, 'Why is not a sign sent down to him?'" meaning: why was a sign not sent down upon him, upon Muhammad? "Say, 'Indeed, Allah is able to send down a sign, but most of them do not know.'" And they are the polytheists.
﴿وَقَالُوا لَوْلَا﴾ هلا ﴿نزل عَلَيْهِ﴾ على مُحَمَّد ﴿أيه﴾ ﴿قُلْ إِنَّ اللَّهَ قَادِرٌ عَلَى أَنْ يُنَزِّلَ آيَةً وَلَكِنَّ أَكْثَرَهُمْ لَا يعلمُونَ﴾ وهم الْمُشْركُونَ.