Each side of the cube has numbers 1,2,3,4,5,6. Thus number of events possible when you roll the dice is 6. Now the probability of getting the 6 is:

P(x=6)=1/6

Thus we know Complement of probability of an event is nothing but 1-Probability of the event.

Thus probability of not getting 6 is equal: 1-P(X=6) = 5/6

I hope this answer helps. All the very best for your future endeavors!