Check for a Windows PC using S-mode, turning off S-mode
Microsoft Windows has a restricted mode of operation for security and safety called S-Mode. From Microsoft "Windows 10 in S mode is a version of Windows 10 that's streamlined for security and performance, while providing a familiar Windows experience. To increase security, it allows only apps from the Microsoft Store, and requires Microsoft Edge for safe browsing."
To check if your PC is using S-Mode go to Settings > System > About. On the About page, scroll down to the “Windows Specifications” section. If you see the words “in S mode” to the right of the Edition entry, you’re using an S Mode PC. If you don’t, you’re not using S Mode.
Microsoft's information about S-Mode is available here Windows S-Mode
This restricts applications to the ones available in the Windows Store.