Hi James,
Open the Settings App
Go to System - About
Is Windows 10 S reported there?
If so your version does not allow installing 3rd party applications . . .
The good news is that Microsoft have realised that Windows 10 S was not such a great idea and are providing a way to swap over to another version of Windows for Free:
Open the Settings App
Go to Update and Security - Activation
Click Go to Store.
That will take you to a "Switch to Windows 10 Pro" page
Click the install button, labeled "Free," to begin the installation process.
Be sure to save all of your data before clicking the "Yes, let's go" option to begin the upgrade