Question

can electric field lines inside an insulator exists? explain what does that mean

Answer 1

Electric field don't exist inside conductors ever. Electric field lines end or are deflected where there is a charge, and the charge in a conductor can only exist on the conductor's surface as a result of Gauss's Law.
These lines are always perpendicular to the conductor surface. In insulating material the electrons are locked up in their atomic orbits so they are not free to distribute themselves throughout the material in response to any internal fields. Therefore in insulators internal electric fields can exist.