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 increase security and performance, Windows 10 in S mode runs only apps from the Microsoft Store. If you want to install an app that isn't available in the Microsoft Store, you'll need to permanently switch out of S mode. There's no charge to switch out of S mode, but you won't be able to turn it back on.
On your PC running Windows 10 in S mode, open Settings > Update & Security > Activation.
Find the Switch to Windows 10 Home or Switch to Windows 10 Pro section, then select the Go to the Store link. Note: Don't select the link under Upgrade your edition of Windows. That's a different process that will keep you in S mode.
On the page that appears in the Microsoft Store (Switch out of S mode or a similar page), select the Get button. After you confirm this action, you'll be able to install apps from outside the Microsoft Store.