croak vs complain

croak is a noun but complain is not a noun.

Word	Noun	Adjective	Verb	Adverb
croak	Yes	No	Yes	No
complain	No	No	Yes	No

As verbs, complain is a hypernym of croak; that is, complain is a word with a broader meaning than croak:

Other hypernyms of croak include kick, kvetch, plain, quetch, sound off.

croak (noun)	complain (noun)
a harsh hoarse utterance (as of a frog)

croak (verb)	complain (verb)
pass from physical life and lose all bodily attributes and functions necessary to sustain life	express complaints, discontent, displeasure, or unhappiness
make complaining remarks or noises under one's breath	make a formal accusation; bring a formal charge
utter a hoarse sound, like a raven