The sample space is not 36. If we consider order of the dice as important then the size of the sample space is 36
but that is not the sample space. The sample space consists of all pairs of numbers 1-6, i.e.
(1,1), (1,2) ... (1,6), (2,1), (2,2) ... (6,6)
Usually when rolling dice the order of dice is not considered important so it would be more accurate to describe the sample space
as all pairs of number 1-6 where the first number is greater than or equal to the second number (or vice versa).
In this case the size of the sample space is 6+(6*5)/2 = 21
(1,1), (2,1), (2,2), (3,1), (3,2), (3,3), (4,1)... (4,4), (5,1) ... (5,5), (6,1)... (6,6)