If you can't perform the prescribed number of chin-ups, your job is to do as many as you can. If doing even one is a struggle, there are a few things you can do to build the necessary strength.
First, use a box or bench to jump to the top of the rep, then slowly lower yourself down. This will teach you how to use your lats and middle back to perform the movement instead of your biceps. If you can handle these reverse chin-ups, keep at them. Eventually, you can try a real one.
You can also utilize bands. Hang one end of the band from the top of the bar, and put your foot or knee into the other end. The band should take enough weight off so that you can perform the reps. I prefer bands to pull-up machines because the machines may not put you in the right position to learn how to pull. If, however, you have no other choice, use a machine.