John Gill Commentary Deuteronomy 18:22

"when a prophet speaketh in the name of Jehovah, if the thing follow not, nor come to pass, that is the thing which Jehovah hath not spoken: the prophet hath spoken it presumptuously, thou shalt not be afraid of him." — Deuteronomy 18:22 (ASV)

When a prophet speaketh in the name of the Lord
Says he comes from God, is sent by him, and has a commission from him to say so:

if the thing follow not, nor come to pass ;
as the prophecy of Hananiah, (Jeremiah 28:3) that is the thing which the Lord has not spoken; or otherwise it would have come to pass, unless when a condition is either expressed or implied, as the repentance or disobedience of a people; see (Jeremiah 18:7–10)

but the prophet has spoken it presumptuously ;
in a bold and daring manner, with great impiety and impudence, out of his own head and heart, being a mere device and imagination of his own, which, not having the fear of God, he delivered as coming from the Lord:

you shall not be afraid of him ;
not only to reprove him for his wickedness, but also to punish him for it; showing no regard to the high character he assumes, nor to the great pretensions he makes to sanctity, knowledge, and familiarity with God.