Microsoft launched the Windows Sandbox feature with the release of Windows 10 Insider Preview Build 18305. It’s a temporary desktop environment that lets you run untrusted software on your system without the fear of it impacting the system. You can enable Windows Sandbox from the Windows Features setting.
While the new Windows Sandbox has been working fine for Windows 10 Insider users, a recent cumulative update (KB4483214) seems to have caused Windows Sandbox to not open/launch on some systems. Microsoft has acknowledged the issue and is working on a fix for the same, but if you want to fix the issue yourself, simply uninstall the KB4483214 update from your system and Windows Sandbox will open as usual again on your PC.
To uninstall the KB4483214 update, go to Settings » Update & Security » click “View Update History” » click “Uninstall updates,” then select the KB4483214 update and uninstall it.
That’s it. After uninstalling the faulty update, restart your PC and try to open the Windows Sandbox. It should work.