A spherical shell of radius R is carries a uniform surface charge q. Take the reference point at ∞

What is the electric field at r<R?

zero q/4πεo(r2 + R2 )  q/4πεoR2   q/4πεor2

zero

The potential inside the spherical shell is constant. Thus, there is no potential gradient which causes the field inside the shell to be zero.