Assertion : The electric field due to a discrete charge configuration is not defined at the locations of the discrete charges.

Reason : For a surface charge distribution, electric field may be discontinuous across the surface.

The electric field due to a discrete charge configuration is not defined at the locations of the discrete charges because by coulomb's law the magnitude of electric field is inversely proportional to square of distance from the discrete charge, as the $r\to 0, E\to \infty$.

Considering a spherical shell with a surface charge distribution, The magnitude of the electric field due to a charged spherical shell is zero inside it, maximum on its surface and then keeps decreasing as 1/r². The electric field across its surface is discontinuous.

Both assertion and reason are true but reason is not the correct explanation of assertion.