Mark 9:29

And he said unto them
Matthew, besides the following reason, assigns another, as given by our Lord, why they could not cast out the foul spirit, which was their unbelief; of which they were guilty in some sort, as well as the Jews, and the father of the child; but Mark omits it, and only relates this as the reason;

this kind can come forth by nothing, but by prayer and fasting;
which they had not observed; (See Gill on Matthew 17:21).