[Baron Grazic]

Population being 0 would be the problem.
You must have a Population of 1M or higher before you can build anything on a Planet.
If you give a Coloniser a "Colonise that planet" order it will automatically load population, move and colonise.
If you give it a move order or Warp order it will not load population, so remember to manually add them first.
I remeber falling for that when starting.