If you are doing the critical angle, you have to go from the denser medium. So in this one, you have to do
Refractive index = 2.03/1.96 = 1.036
If you think about it, the refractive index has to be bigger than 1 because the light is slowing down.
Then, you can use sin(i) = 1 / 1.03
i = sin-1(1/1.03) = 75º
Remember, in critical angles you have to go from the denser medium and that sin(r) = 1 because r = 90.