Hi
On Some planets for whatever reason Planetary soil upgrade is on the build list but can't be placed, later on though other terraforming options can be placed.
in this case this is Class 14 paradise planet.
This bug if it is one seems to happen on the higher class of planets.
Also I find it hard to know which terraforming upgrades can be placed where, as when I have a few of them in the list, they have different salvageable placement tiles available