"s" mode is Microsoft's answer to Apple. It only lets you install software from Microsoft that Microsoft deems safe. If you turn it off it cannot be turned back on but it then will let you install software of your choice. I recently went through that to install AVG free on a computer. https://support.microsoft.com/en-us/windows/switching-out-of-s-mode-in-windows-4f56d9be-99ec-6983-119f-031bfb28a307 a search of how to turn off s mode will give you all you need to know. | 
What does this error message mean ?
