If the game tells you to build certain defence structures (such as Drone Spawners, Tank Spawners or Wedge Guns) but you are unable to do so, please note the following:
Certain defence structures share the same building slots.
For example: The number of Spawners that you are able to build in the game does include both Tank and Drone Spawners. This means that if you have a total of three Spawner slots available and you have already built three Drone Spawners, there is no more room for a Tank Spawner.
These mechanics work similarly for all defensive emplacement such as Wedge Guns.
These building slots are represented by white icons (bullet, drone, missile, and Plasma) next to each of the defensive structures in the Build Menu.