Short Answer 
To fix the ‘This version of Vanguard requires secure boot’ error, restart your computer and enter the BIOS/UEFI settings, typically by pressing a key like F2 or Del. Enable Secure Boot in these settings and ensure the boot mode is set to UEFI, then save your changes and restart your computer. This should allow Vanguard to recognize that Secure Boot is enabled and let you launch the game.
Step 1: Access BIOS/UEFI Settings
To begin fixing the error message ‘This version of Vanguard requires secure boot,’ restart your computer and enter the BIOS/UEFI settings. This is typically done by pressing a specific key such as F2, F10, Del, or Esc right after turning on the computer. Make sure to act quickly, as the time window to enter these settings is brief.
Step 2: Enable Secure Boot
Once inside the BIOS/UEFI settings, navigate to find the option related to Secure Boot. This feature ensures your PC boots only with trusted firmware, which is necessary for Vanguard to operate. In this step, check if Secure Boot is enabled; if not, turn it on. If necessary, adjust the boot mode to UEFI instead of Legacy or CSM.
Step 3: Save Changes and Restart
After enabling Secure Boot and making any required adjustments, make sure to save your changes before exiting. Your computer will then restart with the new settings in effect. If you’ve followed these steps correctly, Vanguard should now recognize that Secure Boot is enabled, allowing you to launch the game without any issues.