You might be able to get by with DirectX9 for the Classic mode only, as long as other system components are not too out of date. However, Reforged mode requires DX11.
The command argument -graphicsapi direct3d9 should work. You can enter it right in the launcher here:
On a side note: if you attempt to install DX11 separately, if your GPU/drivers do not support it, then it will not get used. It will just default to the highest DirectX it can use.