How to enable DX11 in cryengine?

Its the default mode.

r_Driver = DX11

As lavizh already pointed out Directx 11 is used by default.
You can find more information on other API's supported in CRYENGINE in our docs.
But the quick way of switching between DX11 / DX12 / Vulkan would be changing the r_Driver to DX11, DX12 and VK

Quick note, for 5.6 only DX11 will work in the Editor.