Going to be a bit pedantic here but you absolutely can have 0 Voltage. Voltage is the potential energy created by a difference in charge between 2 points. If you have a mass with sufficient free mobile charge, when you inject additional charge into it, it will rearrange the charge within itself to cancel out in injected charge resulting in 0 Volts of potential. This is the principal that a ground connection works on and why you don't have voltage inside of metal conductors, just on the surface.