In short
Matthew 17:21 is not in most early manuscripts, but many translations, like the King James Version, include the clause but this kind does not go out except by prayer and fasting,
because this verse is in the majority of later manuscripts.
Matthew 17:21 is missing from a number of early manuscripts, although many translations, such as the King James Version, include the phrase but this kind does not go out except by prayer and fasting.
1
While it is absent from the oldest manuscripts, this verse is present in the majority of later ones.2
20 He said to them, “Because of your little faith. For truly, I say to you, if you have faith like a grain of mustard seed, you will say to this mountain, ‘Move from here to there,’ and it will move, and nothing will be impossible for you.”