Trying to purchase Battle of Stalingrad

[LJW61]

When I go to the cart to purchase BoS standard I cannot complete purchase, I see this ;      Is this an add-on, do I need to buy a 'base game' first?

 

You can purchase this product set only in the form of gift keys due to one or more following reasons:
- the cart contains incompatible product- the cart contains products incompatible to the ones already activated on your account
- the cart contains products, already activated on your account
- the cart contains products that require the base version of the game

 

[oFlyingDutchman]

Are you trying to buy it from Steam? If so, do you own any other Great Battle products? 

[-DED-Rapidus]

@LJW61, do you have previously purchased products?