Enable SecureBoot when already installed

This web page should help. And notice that one of the conditions is that SecureBoot must be enabled after the OS is installed.

@TheObsolete  As long as Windows was installed in UEFI mode (if your OS disk uses GPT rather than MBR, it's in UEFI mode), then you can enable Secure Boot by simply going into the BIOS Setup and enabling the option.  If your system was set up in Legacy BIOS mode, you'll need to switch it to UEFI mode first.  Microsoft has an in-place conversion utility called MBR2GPT that can help with that.  It converts the disk to GPT and also creates the necessary additional partitions.

@JOcean  You don't have to wait until after installing an OS to enable Secure Boot, because you don't have to disable it in order to install an OS.  Windows 8 and newer AND the corresponding WinPE/RE kernels (i.e. WinPE 4 and newer) all support Secure Boot.

Thanks for the information. Always appreciate your input!