Before mounting the DMG, right-click it and select "Open" rather than double-clicking. This bypasses Gatekeeper issues if macOS says the developer cannot be verified.
Because this is a .dmg file distributed digitally, security is paramount. Follow this protocol to avoid the "App can’t be opened because Apple cannot check it for malicious software" error. Native-Instruments-Traktor-Pro-Plus-3.8.0.46.dmg