Why the Mig21 don't use the ailerons like control surfaces on delta wings at landings or take off?
I'm no expert on the plane, but wikipedia claims the Mig-21 has separate flaps and ailerons.


  On the trailing edge there are ailerons with an area of 1.18 m², and
  flaps with an area of 1.87 m².


The common reasons for not combining flaps and ailerons are that it reduces aileron response and increases the chance of stalling the wing tip (which results in a violent roll in the direction opposite to what you want).

As the Mig-21 has a separate horizontal tail for pitch control, there's no need to use elevons.