Restart PC and boot into your BIOS. Watch the Tutorial if required.
Disable CSM in BIOS. If you cannot explicitly find CSM, then just make sure BIOS is in UEFI mode. If BIOS is in UEFI mode, then CSM is automatically disabled.
Enable RAID in BIOS. Go to Advanced -> Integrated Peripheral -> SATA Mode -> RAID (by default it will be AHCI). After enabling RAID, Save and Rebootback into BIOS.