Aspire E1-571G Security boot mode remain greyed and STANDARD

Hi all,

i'm trying to boot an usb stick with win10 on it (made with Rufus). I've switched from UEFI to Legacy under the boot tab, but when the usb boot starts the screen show some artifact and nothing else. I've tried to use different windows images with Rufus but this hasn't solved the artifact problem. I've noticed that, in BIOS, even if i switch to Legacy under the boot tab, the "security boot mode" under the security tab remain greyed and set On STANDARD. I can't change that mode even if i set a superuser password. What should i do now?