Electric field strength

You have said that it is a "conductor". That is to say, inside which there are electrons that can be moved freely under the action of electric field.

Now, if there existed an electric field inside a conductor, those free electron would move due to the action of the field. Movement of electrons would alter the potentials at different parts of the conductor until all parts are equlaized, i.e. at the same potential. Under such situation, electro movement would then stop.

The equalization of potential on a conductor also leads to the result that the electric field inside it vanished.
I don't know how your calculation is performed. Perhaps you have neglected the point that all cahrges must reside on the surface of the conductor. By applying Gauss Law, if we choose a Guassian surface to be a sphere centred at the centre of the spherical conductor, the Gaussian surface then encloses no charge. It indicates that the electric field on the Guassian surface is zero. As the chosen Gaussian sphere can be of any arbitrary radius up to, but not equal to, the radius of the spherical conductor, this shows that the electric field doesn't exit inside the conductor. A consequence of no electric field implies a constant potential inside the conducting sphere.