During the DAoT, humans were able to terraform:
Percipre was once a famous garden world. Terraforming created a lush paradise, stocked with an incredible variety of plant species from around the galaxy. With the failure of the Terraforming Engines, the planet's population constructed vast glasshouses to protect the collections from the ever harsher conditions, and now only the hardiest of xeno-flora survive outside of their protection.
Terraforming: Percipre was extensively terraformed some time before Imperial records pertaining to the Calixis Sector began - possibly as far back as the Dark Age of Technology. Enormous machines provided a lush environment famed for its fertility. However, in a later age, the knowledge to maintain the Great Engines was lost, and the planet's inhabitants were forced to turn to more primitive means as their world's atmosphere began to leach off in to space. As a result much of Percipre's surface is covered by vast gas-tight glasshouses constructed from great panes held in ornate wrought iron frameworks. Within these gothic edifices, most large enough for a Titan to walk erect, row after row of foodstuffs grow in hydroponics beds, tended by massed ranks of faceless servitors.
From the article on Percipre on Lexicanum
It mentions that the knowledge to maintain the Terraforming machines was lost. But with how vast the Imperium is, this does not necessarly mean that the knowledge is lost in general. It may simply be that the Mechanicus considers this world not important enough to dedicate specialized adepts and magi to maintain them.
Does the Imperium still have the knowledge to create and maintain Terraforming machines or has this been utterly lost?