why is it said that snell's law fails when angle of incidence is 0

Hi Theertavm!

When the angle of incidence is zero, that is, normal to the surface, then the ray passes without any deviation. In this case, the angle of refraction is also zero. As the division by zero is not defined, we cannot find the refractive index in this case. This leads to the failure of the law, in this situation.

Cheers!!