Why you can you do n-12 but not 12-n?

The subtraction is not commutative, it means that n - 12 is not the same as 12 - n, except when n is 12. For example, suppose that we have a statement such as n - 12 = 12 - n. Can we find a value for n which will satisfy our statement?
n - 12 = 12 - n (add n and 12 to both sides)
n + n - 12 + 12 = 12 + 12 - n + n
2n = 24 (divide by 2 both sides)

n = 12
Check:
n - 12 = 12 - n
12 - 12 =? 12 - 12
0 = 0 True

But, for all other values of n, the statement is not true. Let's say that n = 20
Does 20 satisfies the statement? Check:
n - 12 = 12 - n
20 - 12 =? 12 - 20
8 = -8 False

Hence, we cannot write n - 12 as 12 - n for n ≠12.