KJV: And Jesus said unto them, Because of your unbelief: for verily I say unto you, If ye have faith as a grain of mustard seed, ye shall say unto this mountain, Remove hence to yonder place; and it shall remove; and nothing shall be impossible unto you. Howbeit this kind goeth not out but by prayer and fasting.

NIV: He replied, “Because you have so little faith. Truly I tell you, if you have faith as small as a mustard seed, you can say to this mountain, ‘Move from here to there,’ and it will move. Nothing will be impossible for you.”

ESV: 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.”

CSB: “Because of your little faith,” he told them. “For truly I tell you, if you have faith the size of a mustard seed, you will tell this mountain, ‘Move from here to there,’ and it will move. Nothing will be impossible for you.”

Explanation

This same story is told in Mark 9:29 where the sentence in question does occur, and a scribe inserted it into Matthew 17 after v 20.

NIV: Mark 9:28 After Jesus had gone indoors, his disciples asked him privately, “Why couldn’t we drive it out?” 29 He replied, “This kind can come out only by prayer.”

Result

The omission of Matthew 17:21 loses nothing because it is in Mark 9:29.