SPCR

I just built a new PC and the LAN light is not lighting up when plugged in. It works on Xbox/Laptop.

Keep trying to install newest drivers from Manufacturer website for Asrock Fatality AB350Gaming K4 motherboard and get the same error, "The Realtek Network Controller was not found. If Deep Sleep Mode is enabled Please Plug the Cable."

I cannot get Realtek to show up in the Network Adapter devices so I cannot turn off the mode. Any help please?

That is unfortunately most probably a hardware problem that's slipped through the production tests.
Make it a warranty case (RMA) by the routines applicable to your location.