challenge vs ask for

challenge is a noun but ask for is not a noun.

Word	Noun	Adjective	Verb	Adverb
challenge	Yes	No	Yes	No
ask for	No	No	Yes	No

As verbs, ask for is a hypernym of challenge; that is, ask for is a word with a broader meaning than challenge:

Other hypernyms of challenge include bespeak, call for, quest, request.

challenge (noun)	ask for (noun)
a call to engage in a contest or fight
questioning a statement and demanding an explanation
a formal objection to the selection of a particular person as a juror
a demand by a sentry for a password or identification
a demanding or stimulating situation

challenge (verb)	ask for (verb)
issue a challenge to	express the need or desire for
take exception to	increase the likelihood of
raise a formal objection in a court of law
ask for identification