Correct spelling, explanation: the word impact originates in Latin impactus (past participle form of impingere). Inpact is an incorrect form, probably mistaken because of the consonant p (usually negative prefix im- is added to nouns starting with p, however in this case impact is a separate word).

Definition of impact:
noun, a forceful contact; influence on
That accident had an impact on me. I changed for the better, I think.
The impact on the construction is too big, we have to change the project.
verb, to make contact, usually forcefully

Incorrect spelling, explanation: there is no such form as inpact; it is a misspelling probably due to p (normally a negative prefix im- is added to words that begin with p, but impact is a separate word, not a prefix with a noun). Therefore the correct form is impact.