Mac App Store apps and VPP device assignment are available in the latest version of the Apps page. Under MDM > Apps, click 'Try the new version!', and you'll see the option to add macOS App Store apps, as well as import licensed applications from your VPP, along with a whole host of UI enhancements. For troubleshooting packages and DMG installs with software installer, check device logs as described here, and create a support ticket for further assistance as necessary.
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