kick off vs kickoff

kick off is not a noun while kickoff is a noun.

Word	Noun	Adjective	Verb	Adverb
kick off	No	No	Yes	No
kickoff	Yes	No	No	No

kick off (noun)	kickoff (noun)
	(football) a kick from the center of the field to start a football game or to resume it after a score
	a start given to contestants
	the time at which something is supposed to begin

kick off (verb)	kickoff (verb)
commence officially