There’s been a lot of talk in recent days about the TV ads Democratic candidate Elizabeth Warren is running in her bid to unseat Republican Senator Scott Brown. But it’s the ads she isn’t running that we want to see. We’re told that Oscar-winning director Errol Morris has approached the Warren campaign on two occasions with an offer to make commercials — for free — on Warren’s behalf. But we hear the campaign hasn’t taken him up on the offer, or even directly responded to the invitation. Morris, who lives and works in Cambridge, is perhaps best known as the director of such documentaries as “The Thin Blue Line” and the Academy Award-winning “The Fog of War.” But he’s also an in-demand director of TV commercials, including spots for companies ranging from Adidas to Nike to Nasdaq. (Our favorites are his ads for Miller High Life.) So why isn’t the Warren camp jumping at the chance to work with an avid supporter who happens to be a celebrated filmmaker? “Unfortunately, as nice as that would be, we can’t accept ads for free,” someone with the campaign told us Friday. “It would violate Federal Election Commission rules. . . . But we do appreciate his support.” At the moment, Morris is busy promoting not a movie but a book. Called “A Wilderness of Error: The Trials of Jeffrey MacDonald ,” the book is an examination of the case against MacDonald, the Green Beret doctor who was convicted in 1979 of murdering his wife and two daughters.