I am unable to install Mac OS on a used IMac I purchased

Like many others, I ran into the dreaded “The recovery server could not be contacted” message when trying to do a restore of Mac OS High Sierra for a 2010 13 inch MBP that I was selling (yes I know it’s worth almost nothing, but someone wanted to buy it from me)…

https://discussions.apple.com/thread/254871592?answerId=259137215022#259137215022

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 for 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.

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Clean Install of High Sierra: Multiple Fa… - Apple Community

Did the seller or you follow all the steps in What to do before you sell, give away, or trade in your Mac