Sometimes the planet doesn't have any easily-terraformed spots.  Especially true on planets that start out high quality.

Some planets start with high quality and can't make use of it

Guess those 'perfect' planets have national park areas that are untouchable

Check out the 6.2 patch notes that just came out, if you can't use them its grayed out

It isn't a compete fix for this issue but its getting there lol

its not a bug apparently, if a class of a planet is high enough then it might naturally not have land that it can improve. Which makes sense considering the techs are more just to improve crappy planets not make every great planet better. Kinda an extreme example would be if you colonized the moon, and then got a tech for Ocean habitats.. wouldn't be right to say "oh i got the ocean tech i should be able to build it on this dry rock of a planet". I mean its not the same thing, but you know what i mean.