can't use terraforming in capital level 16 planet

It happens, that is because terraforming improvements have quality level corresponding to to quality of unconverted tile and if there is no tile they can convert that improvement is effectively wasted. I can't give you exact details since I never look at those numbers closely and only thing I can recommend is to use terraforming in order you get them in tech tree to minimalise this effect. This is actually a problem on big planets like 15+, I don't remember having it on smaller ones.

so the reason is my planet is level 16 and i can't get any more terraforming but i remember in my previous play i could get terraforming even on level 20 planets although fewer  terraforming but i could still get some and also this is the first terraforming in the tech tree i have unlocked could this be a bug 

its not about planet size but quality of tiles, you will be able to use more advanced terraforming techs

oh now i get it 

i haven't played this game for almost  5 months and i have forgotten some things already 

anyway thanks karlfranz-pl

it is pretty annoying when cycling through planets and looking for terraforming tiles, but some of them are both available and unusable.  It would be nice (from a QoL and UI standpoint) if a planet with no available tiles for a terraform, would at least grey out those improvements.

Or all the Terraforming improvements always work on all planets.

The game design is a bit wobbly, when the player research for a technology and cannot use the bonuses.

A save file for the bug here.

Random.

If the game is well designed, the random range can work in any case.  

Quoting Unknown_Hero, reply 8

If the game is well designed, the random range can work in any case.  

It could have been better solved/explained in game but considering all this is non issue and you are wasting time.