All of you wrong! The subject of the verb "miss" is 'that"; from the clause preceding it: "..that are supposed to install DNS on every PC, but (that) missed some." Of course, the "that" in the second clause refers to the "environments" of the first. Here is the sentence in full: This is a common misconfiguration for environments that are supposed to install DNS on every PC, but (that) miss some. That should clear it up.

environments

'Environments' is the subject.

environmentalists is the subject if i understood it right

hey environments or environmentalists