HOW DOES Fe SHOW +6 O.S BUT NOT +5, SIMILARLY Co DOES NOT SHOW +5 O.S ?

dear Student,

Oxidation state is basically the number of electrons accepted or given by atom in order to get stable electronic configuration. 

Fe electronic configuration : $\left[Ar\right] 3d^6 4s^2$
so electronic configuration which has fully filled or half filled outermost shells are most stable. therefore Fe has 6 electrons in d orbital, so it either loses 1 electron and form half filled shell, or it looses all the six electons and attain stability.

similarly Co, it has the electronic configuration.$\left[Ar\right] 3d^7 4s^2$
so it loses2 electrons or 7 electrons.

hope this concept is clear to you

Regards