Don't change BIOS to LEGACY.

Windows can be installed in two ways: Legacy-MBR or UEFI-GPT
To install as Legacy-MBR you must boot the installation drive as Legacy
To install as UEFI-GPT you must boot the installation drive as UEFI.

As you have a new computer, you should install as UEFI-GPT

During POST, press F? to launch the boot menu. You will see two options for the ODD drive. ODD UEFI (Name) and ODD (Name). Select ODD UEFI (Name).
Go to install and delete ALL partitions on the SSD till you have one and only one unallocated space and then proceed.