Windows 10 Apps Can't Open or Windows 10 Apps are Greyed Out

Windows 10 Apps are greyed out or when clicking a Windows 10 App such as calculator or weather, the following error message is encountered:


Resolution:

Open Regedit
Navigate to:
HKEY_LOCAL_MACHINE\SOFTWARE\Microsoft\Windows\CurrentVersion\AppModel\StateChange\PackageList

Delete all these keys listed under this key

Restart Windows