Which environments are you referring to? Venus is the closest planet to Earth. The Moon is the only significant body between the two.
And by 'we've failed to terraform', did you mean 'we haven't tried yet'?
And by 'we've failed to terraform', did you mean 'we haven't tried yet'?
California's Central Valley project is an example, and China has some that are even larger.
Plus, there's no life that we know of on Venus. A bacterial population capable of living, evolving, and growing exponentially in that environment would be a huge change.