why cyclo hexene cannot exhibit geometrical isomerism?

You have to visualize these two moleculkes in three dimensins. benzene is flat so that two chloro atoms would also be aligned in the benzene plane. Since all carbon atoms ar sp2 hybridized all the substituents are aligned in the flat trigoal planar shape.

In cyclohexane all carbon atoms are sp3 hybridized and are tetrahedral. Therefore if two chloro atoms are attached they could wither be facing each other (cis) or point away from each other (trans). IIf you have a model kit you should build the two.