Well 12 x 21 = 252 so the smallest possible difference is ZERO.
If you want the smallest possible POSITIVE difference, let's write out the multiples
12 24 36 48 60 72 84 96 108 120 132 144 156 168
21 42 63 84 105 126 147 168 This repeats every 4th multiple of 21 (or 7th multiple of 12)
The LEAST POSITIVE difference is 3 it appears .