That's because of the teaching language in use, it's probable that you're working with a student language with certain limitations. To solve the problem, make sure that this line is at the beginning of the file:
#lang racket
And choose the option "Determine language from source" in the bottom-left corner of DrRacket's window. Now this should work as expected:
(cons 1 2)
=> '(1 . 2)