بَلْ قَالُوا أَضْغَاثُ أَحْلَامٍ (they said, [ The Further Qur'an is ] a mixture of hotch-potch dreams. - 21:5.) Dreams having an element of personal and shaitanic thoughts are called أَضْغَاثُ أَحْلَامٍ. That is why this term (أَضْغَاثُ أَحْلَامٍ ) has been translated as "hotch-potch dreams". What it means is that in the first place the unbelievers called the Qur'an as magic, then they described it as a collection of disturbed dreams, and then they said it was a forgery and fabrication against Allah Ta’ ala to call it His words; and finally they said that he ﷺ was a poet and the Qur'an represented his poetic compositions.
فَلْيَأْتِنَا بِآيَةٍ (So let him bring to us a sign - 21:5) It means that the unbelievers demanded from the Holy Prophet ﷺ certain specific miracles to be shown as a proof of his being a real Prophet. In response to this demand, Allah Ta` ala said in this verse that the same demand for miracles was also made by the people in the past from other prophets, and when their request was met, it was of no avail. They did not submit to Allah even after witnessing the miracles of their choice. And Allah has decreed that people who do not submit to Him even after they have seen the miracle of their choice are subjected to His wrath even in this world and are destroyed. Allah, in His Divine Mercy and in view of the honoured position of the Holy Prophet ﷺ had granted to his Ummah immunity from the terrible punishment which is the inevitable fate of those people who defy the Will of Allah. It was not, therefore, considered desirable to show them miracles of their choice because if they, like the earlier people, persisted in their unbelief even after seeing those miracles, they too would invite the wrath of Allah Ta` ala.