Enable VT Technology in BIOS.
Make sure proper boot image of server is selected, 1st boot device is CD/DVD and 2nd boot device is hard disk, also make sure virtual hard disk is created.
I am facing the same issue while installing any operating system in HyperV. tried all the possible steps and checked the boot sequence. On Virtualbox by oracle is installing windows fine but not on HYperV.
Select legacy instead of uefi.
I was also facing same issue
first create network adapter in Hyper-V then re-install everything with network adapter it will work.