Apple just released iOS 12 on Monday, so it shouldn’t come off as much of a surprise that there isn’t a public jailbreak solution available for it at this time.
But that hasn’t stopped Cydia creator Jay Freeman (Saurik) from releasing an update for the Cydia Impactor side-loading tool with official iOS 12 support. As of this writing, the latest version of the utility is 0.9.46.