Answer

Please set W0indows 10 WHQL Support to enabled first, then "Secure Boot" option would appear below. 

Models

B250I GAMING PRO AC, B350 GAMING PRO CARBON, Z370 GAMING M5, B350 PC MATE, H270 GAMING PRO CARBON, X299 SLI PLUS, Z270-A PRO, H270 GAMING M3, Z370M GAMING PRO AC, B250M BAZOOKA, B350M MORTAR, H270-A PRO, Z270 GAMING PLUS, A320M BAZOOKA, B250M BAZOOKA PLUS, X370 KRAIT GAMING, B350 TOMAHAWK ARCTIC, B450M BAZOOKA PLUS, X299 TOMAHAWK ARCTIC, X299M GAMING PRO CARBON AC, Z270 GAMING PRO CARBON, Z270 TOMAHAWK ARCTIC, B450M BAZOOKA, H270M MORTAR ARCTIC, X299 XPOWER GAMING AC, B350 KRAIT GAMING, B450 TOMAHAWK, H270M BAZOOKA, X370 XPOWER GAMING TITANIUM, X470 GAMING M7 AC, A320M PRO-VD/S V2, Z270 XPOWER GAMING TITANIUM, Z370 GODLIKE GAMING, B250M MORTAR ARCTIC, X299 GAMING PRO CARBON AC, X299 RAIDER, X299 TOMAHAWK, Z270 GAMING M5, X370 GAMING M7 ACK, B250 GAMING PRO CARBON, B250 PC MATE, X299 GAMING M7 ACK, X470 GAMING PLUS, Z270M MORTAR, B350M GAMING PRO, X299 GAMING PRO CARBON, B250M PRO-VDH, B450M PRO-VDH, B450M PRO-VDH V2, Z270 SLI, A320M GAMING PRO, B250M PRO OPT BOOST, Z270 SLI PLUS, Z270-S02, A320M GRENADE, A320M PRO-VH PLUS, B350 TOMAHAWK PLUS, B350M MORTAR ARCTIC, Z270 GAMING M3, Z370 KRAIT GAMING, B350M PRO-VD PLUS, X299M-A PRO, Z270 GAMING PRO, B350M PRO-VH PLUS, Z270 GAMING M6 AC, B250 KRAIT GAMING, H270 PC MATE, Z270 CAMO SQUAD, Z370 TOMAHAWK, B350I PRO AC, B350M BAZOOKA, X370 GAMING PLUS, Z370 GAMING PRO CARBON AC, A320M PRO-VD PLUS, Z370M MORTAR, B250 GAMING M3, B250I PRO, B250M BAZOOKA OPT BOOST, B450M PRO-M2, Z270 TOMAHAWK, Z370-A PRO, X470 GAMING PRO, A320M PRO-VD/S, B250M PRO-VH, B450I GAMING PLUS AC, X470 GAMING PRO CARBON, Z370 PC PRO, Z370 SLI PLUS, A320M PRO-M2, B250M PRO-VD, H270I GAMING PRO AC, X370 GAMING PRO, Z270 PC MATE, B450-A PRO, X299 TOMAHAWK AC, Z270 TOMAHAWK OPT BOOST, Z370 GAMING PLUS, B250M GAMING PRO, B450 GAMING PLUS, X370 GAMING PRO CARBON AC, Z370 GAMING PRO CARBON, Z270 GAMING M7, Z270 MPOWER GAMING TITANIUM, B350 TOMAHAWK, B350M PRO-VDH, X370 GAMING PRO CARBON, Z270I GAMING PRO CARBON AC, Z370I GAMING PRO CARBON AC, B250M MORTAR, B450 GAMING PRO CARBON AC, B450M BAZOOKA V2, B450M GAMING PLUS, A320M PRO-VD, A320M PRO-VHL, B450M MORTAR, X370 SLI PLUS, X470 GAMING PRO CARBON AC, Z270 KRAIT GAMING, B350 GAMING PLUS, B450M MORTAR TITANIUM, H270 TOMAHAWK ARCTIC