Why can’t I build my assault fortress in ac4?

Your structure will need to be built next to an existing alliance building. There may also be a limit to how far away it can be. I have also seen in the past a glitch where the game doesn't allow you to place in some locations even though they should be valid. I do not know if this was resolved. You can try reporting any problems like this to the developers to check if it is a valid restriction or a glitch.