I ran seamless on Windows 10, but I didn't install or update directx.
AFAIK, Windows from Vista to 10 includes dx9, wich is the latest compatible runtime for dx 7, 8 and so.
From dx10 until now, despite a discrete backward compatibility, there's almost no support for APIs as ancient as directx 8.
You could try uninstalling dx12 or install on its side the 9.0x runtime.
By the way, didn't you receive the popup asking to install needed functionality at program launch?