The electric field is $E = -\frac{dV}{dx}$ if V=0, we can not say E must be zero, we say only E may be zero. if $ V \ne 0$, E may be zero when V is maximum or minimum i.e, $\frac{dV}{dx} = 0$ if $E\ne 0$ , V may be zero when two equal and opposite charges separated by a distance then at the midpoint in between the charges field non-zero but potential is zero.
