I can't build fortress II and garrison II even though I have Alliance centre, why?

Assuming you mean the Assault Fortress and Guardian Fortress Buildings from the Doomsday event. These work differently from the virus research and production buildings.

Although there are 4 to build (1 for each alliance center) only the first one of each are unlocked automatically. For the fortress building you have to unlock the others using speciality points in the Speciality skill section of the doomsday menus.

In the blue 'construction speciality section the bottom branch of skill items will allow you to unlock fortress buildings II, then III and finally IV if you have enough skill points to use.

For each doomsday event, once you have set the speciality points for these and placed the buildings you can then respec and change the points to other skills and keep the buildings. But you have to unlock them first before placing them.