﴿ إِلَّآ إِبْلِيسَ ٱسْتَكْبَرَ وَكَانَ مِنَ ٱلْكَٰفِرِينَ ﴾

“save Iblis: he gloried in his arrogance, and [thus] became one of those who deny the truth.”

(Saving Iblis, he was scornful) he was too proud to prostrate before Adam (and became one of the disbelievers) because of disobeying Allah's command.