Slitherine

How come I can't buy heavy artillery 203 mm 1939 in red storm ? Is there anything to unlock ?

I can see players using it in some AAR's.

Its production ended in late 1941. So you can buy in the 1st Soviet DLC, and import from there if it's still alive, but not buy new later.

This artillery is a Limited Edition... but we can still see this artillery during the Battle of Berlin :

In addition to its regular role as a frontline heavy artillery piece, the Soviets liked to use it for siege warfare in urban combat situations. They used it a lot during the Battle of Berlin, where they would set them up in the streets and use them to demolish buildings that the Germans occupied.

They used 203mm artillery throughout the war, even later in Manchuria
But I agree with Bebro that it shouldn't be available on Red Steel and Red Storm, possibly as an aux unit during scenarios like Berlin or Seelow (only if the player didn't import the core)
The Soviets had quite a lot of this equipment and were not losing it in large numbers in 1941-42, because the heavy artillery brigades operated quite far from the frontlines and were completely motorized. Therefore, I believe that the battle of Moscow should be the last moment when we can buy this unit, and then only as an aux unit.