change this
let email = result["email"] as? String
into
guard let resultNew = result as? [String:Any]
let email = resultNew["email"] as! String
full answer
let parameters = ["fields": "email, first_name, last_name, picture.type(large)"]
FBSDKGraphRequest(graphPath: "me", parameters: parameters).start { (connection, result, error) in
guard let resultNew = result as? [String:Any]
let email = resultNew["email"] as! String
}