Tafsir of Al-Anbiya' 21:39

لو يعلم الذين كفروا حين ﻻ يكفون عن وجوههم النار وﻻ عن ظهورهم

If only those who disbelieved knew (the time) when they will not be able to ward off the Fire from their faces, nor from their backs, meaning, if only they knew for certain that it will inevitably come to pass, they would not seek to hasten it. If only they knew how the torment will overwhelm them from above them and from beneath their feet

لهم من فوقهم ظلل من النار ومن تحتهم ظلل

They shall have coverings of Fire, above them and coverings (of Fire) beneath them. (39:16)

لهم من جهنم مهاد ومن فوقهم غواش

Theirs will be a bed of Hell (Fire), and over them coverings (of Hellfire). (7:41) And in this Ayah Allah says:

حين ﻻ يكفون عن وجوههم النار وﻻ عن ظهورهم

when they will not be able to ward off the Fire from their faces, nor from their backs, And Allah says:

سرابيلهم من قطران وتغشى وجوههم النار

Their garments will be of tar, and fire will cover their faces. (14:50) The torment will surround them on all sides,

وﻻ هم ينصرون

and they will not be helped. means, and they will have no helper. This is like the Ayah:

وما لهم من الله من واق

And they have no guardian against Allah. (13:34)