Just click and use the pop up menu. Or hit space and use arrow keys and enter for menu. The first 3 levels are tutorials that limit where you can build. The text proceeding the level will tell you where you can build. Lol 1 the 2 forward towers
Just watched your video. Game is working as intended. In the briefing screen it says "forward of the installation are 2 free standing towers" those are where you can build. They are the 2 white squares just at the top of the screen away from the rest of the walls. I have now added highlights to these towers and and animated all tutorial highlights. If you download the latest version you should see green pulsing boxes where you can build.