Why the Mig21 don't use the ailerons like control surfaces on delta wings at landings or take off?

The reason to use ailerons as flaperons on delta wings is twofold:

• Delta wings reach maximum lift at a much higher angle of attack, which entails high pitch angle on landing, which is impractical. A way to generate enough lift at a reasonable angle is required.
• With low aspect ratio, space on the trailing edge is very limited, so there is a desire to use all of it.

However, there were several considerations against using flaperons on MiG-21:

• Overall, MiG-21 is still a 'classic' airplane with conventional mechanical (though boosted) controls. It is not a pure delta which would require special controls by definition.
• It is not only classic, but cheap and simple by design. Adding control mixers would complicate the system.
• To compensate for the lift problem at landing, two things were used:
  • Most variants had the 'blown flaps' system, which used compressed air from the engine to blow the boundary layer from above the flaps. The air was injected in the (single) gap of the flap. Like any fighter, MiG-21 had a significant excess of engine power available on landing, and it could be put to use this way.
  • A somewhat higher landing speed was simply tolerated. The landing speed was about 280 km/h (~150 kt). As one of the results, main tyres were wearing out quicker (I heard the typical lifespan was 5-10 landings).*

(*) That said, the contemporary F-104 was reportedly even worse in this regard, without being delta.

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.