The recovery server could not be contacted
How can I install the Mac OS X ?
iMac 21.5″, macOS 10.13
1. Start up from macOS Recovery
Option-⌘-R
Upgrade to the latest macOS compatible with your Mac.
APFS won't work for Sierra, Sierra needs MacOS Extended Journaled. High Sierra won't work fir APFS on earlier Macs.
What has often worked for others is to use this Restore Method instead of CMD+r keys...
Option-⌘-R
Upgrade to the latest macOS compatible with your Mac.
2017 iMac failing to install Sierra in re… - Apple Community