I don't even play Valorant or use Vanguard, but helped a friend with this issue. The most consistent fix for people seeing this message is to enable secure boot from BIOS. It also must be active, if you enter BIOS and see that it is enabled but not active, you have to toggle it off and back on by changing to custom setting, then back to enabled, and when asked restore defaults.
If you don't have an option to enable secure boot, you must first disable CSM, though I don't remember off the top of my head what that stands for and different BIOS may list this in different ways. The M is for management, for what that's worth.
Some computers, especially laptops, have extremely limited BIOS settings. If you happen to have one of those, you just won't be able to play from that machine anymore unless Vanguard finds another way to implement whatever it is they have done that requires this setting.
