[plt-scheme] reference to undefined identifier

hi, i am new and i have a problem.i couldn't find any solution :( i am
sure it is a basic and an easy problem..
i saw these codes in a scheme book but they didn't work.it gives this
problem: reference to undefined identifier: accumulate. i don't know
what that means.what can i do to fix? i use pretty big language.

(define (acronym phrase)
(accumulate word (every first phrase)))
> (acronym '(american civil liberties union))