Building on a planet, The Hateful Machine Unit (HMU) Knights of Rancor is partially correct. When you build marines either HMU's or MU's on a planet all trading is essentially done, you can no longer buy or sell goods to the planet. So, make sure all merchant trade is concluded prior to building MU's or HMU's on a world, once per action per turn.
But, you can still build forts, torps, ABM's if your world's tec will allow and ship yards, as well as building PC's or MC's I've not had any of these fail after building MU's.