You can only enhance/terraform certain tiles and some planets don't have any. There is no way to know which tiles are improvable until you research the techs and try to build the improvement. It's kind of a crappy system and I hope it gets adjusted. 

On all the other planets I have, if I select Soil Enhancement or Terraforming Plant, one or more tiles are highlighted, and I can then choose one of them to enhance.  On this one planet, no tiles are highlighted to allow me to choose.

Here's how I understand it:

All the tiles on your planet are Marked at different levels: usable, salvageable, decent, lesser, almost useless and useless. The usable tiles are available initially. Then, as you unlock the terraforming techs, you gain the ability to terraform one tile of the listed quality or better. There is no guarantee, however, that there will be any salvageable, decent, lesser, or almost useless tiles on your planet. So what is happening in your example is that on your class 15 planet you have no salvageable or decent tiles.

This is the way GalCiv II did it as well, only they let you see the lower quality tiles coded as yellow orange and red, and you were able to improve all of the lower-quality tiles on your planet rather than 1 per tech. The tiles are mapped to match up with the land on the map, so I have found that on higher quality planets there are often no or very few lower-quality tiles because the usable tiles are covering all the land.

Thanks for mentioning this, I discussed it with Paul and there will be tweaks to that system.