insulator does not have free electrons.....so if insulators are rubbed ...there must be no transfer of electrons...how they can be charged by rubbing?

We know that in insulator the electrons are tightly bound by their shell.

When we rub the insulator with other material, their is a kinetic energy gained by the insulator due to rubbing and hence their shell broke.

Therefore charge started flowing.

In this way insulator get charged while rubbing.