Enabling Secure Boot in Lenovo Think systems

You need to set Platform in "User Mode", Secure Boot in "Standard Mode" and Load Setup Defaults.

You could do it by Restoring Factory Keys:

BIOS - Security - Secure Boot - Restore Factory Keys - Enter

BIOS - Restart - OS Optimized Defaults - Enabled

BIOS - Restart - Load Setup Defaults - Enter

Go to BIOS - Main and check if UEFI Secure Boot is ON.