وَقَالُوا۟ لَوْلَا نُزِّلَ عَلَيْهِ ءَايَةٌ مِّن رَّبِّهِۦ ۚ قُلْ إِنَّ ٱللَّهَ قَادِرٌ عَلَىٰٓ أَن يُنَزِّلَ ءَايَةً وَلَـٰكِنَّ أَكْثَرَهُمْ لَا يَعْلَمُونَ Qur’an Al-An'am (6:37)Waq a loo lawl a nuzzila AAalayhi a yatun min rabbihi qul inna All a ha q a dirun AAal a an yunazzila a yatan wal a kinna aktharahum l a yaAAlamoon a
I.e., on Muhammad, to demonstrate that he is really a bearer of God's message.
Lit., "most of them do not know", i.e., that God manifests Himself always - as the next verse points out - through the ever-recurring miracle of His creation.
Sins are all around them, but they do not understand. If they want a particular Sign to suit their gross ignorance, they will not be humoured, for they can always pick holes in anything that descends to their level.