When you turn on your computer, first, the firmware settings are loaded and a disk system check is initiated. In the next step, the operating system is loaded by the Windows Boot Manager. After that, WinLoad.exe loads important drivers to start the Kernel which loads the registry settings and additional drivers and gives control to the system manager process. Now, your UI and the rest of the software are loaded and you see the login screen. The SAFE_OS phase is the stage where the system verifies that everything including the drivers is working fine as expected. During this phase, if anything insecure is found or if any of the signatures are missing, you may get the installation failed in the SAFE_OS phase error.