28 After Jesus had gone indoors, his disciples asked him privately, “Why couldn’t we drive it out?”

Read Mark 9:28 Using Other Translations

And when he was come into the house, his disciples asked him privately, Why could not we cast him out?
And when he had entered the house, his disciples asked him privately, "Why could we not cast it out?"
Afterward, when Jesus was alone in the house with his disciples, they asked him, “Why couldn’t we cast out that evil spirit?”