I believe you should try imaging using a USB cable.
Please take a look at
https://wpilib.screenstepslive.com/s...g-your-roborio
It categorically says that "The roboRIO should only be imaged via the USB connection. It is not recommended to attempt imaging using the Ethernet connection."
Connect a USB cable from the roboRIO USB Device port to the PC. This requires a USB Type A male (standard PC end) to Type B male cable (square with 2 cut corners), most commonly found as a printer USB cable.
Hope it helps.
Chet