Say, "None in the heavens and earth knows the unseen except Allah, and they do not perceive when they will be resurrected."

The One Who knows the Unseen is Allah

Allah says:

قُل لاَّ يَعْلَمُ مَن فِي السَّمَاوَاتِ وَالاَْرْضِ الْغَيْبَ

Say:None in the heavens and the earth knows the Unseen,

Allah commands His Messenger to inform all of creation that no one among the dwellers of heaven and earth knows the Unseen, except Allah.

...
إِلاَّ اللَّهُ

except Allah,

This is an absolute exception, meaning that no one knows this besides Allah, He is alone in that regard, having no partner in that knowledge.

This is like the Ayat:

وَعِندَهُ مَفَاتِحُ الْغَيْبِ لَا يَعْلَمُهَأ إِلاَّ هُوَ

And with Him are the keys of the Unseen, none knows them but He. (6:59)

إِنَّ اللَّهَ عِندَهُ عِلْمُ السَّاعَةِ وَيُنَزِّلُ الْغَيْثَ

Verily, Allah, with Him is the knowledge of the Hour, He sends down the rain. (31:34). until the end of the Surah.

And there are many Ayat which mention similar things.

...
وَمَا يَشْعُرُونَ أَيَّانَ يُبْعَثُونَ

nor can they perceive when they shall be resurrected.

That is, the created beings who dwell in the heavens and on earth do not know when the Hour will occur, as Allah says:

ثَقُلَتْ فِى السَّمَـوَتِ وَالاٌّرْضِ لَا تَأْتِيكُمْ إِلاَّ بَغْتَةً

Heavy is its burden through the heavens and the earth. It shall not come upon you except all of a sudden. (7:187)

meaning, it is a grave matter for the dwellers of heaven and earth.