What causes the "Recovery Server Can't Be Contacted" error on my iMac?

My 2011 iMac will not start properly. It is stuck on the apple/white screen. I start in recovery mode and get Disk Utility. when I got to reinstall Mac OS (in this case High Sierra) when I hit 'continue' it eventually says 'Error "Recovery server can't be contacted'. BTW I have a wifi connection with the internet showing in the top right hand corner of the screen.

[Re-Titled by Moderator]