Trying to reinstall OSX Lion on my old iMac. I get a message that says Item is temporarily unavailable; try again later

Installing from the App Store or recovery mode may be limited by the system time.

https://appuals.com/this-copy-of-the-install-os-x-el-capitan-application-cant-be-verified/

Explains how to deal with that.

Lion can only be installed one of these ways:

On Macs made between July 22, 2011 and July 25, 2012 using command-option-shift-R boot on wired keyboard. Ideally connect the Mac to ethernet for the install, otherwise it might be slow.

On Macs that have 10.6.6 to 10.6.8 installed from the App Store.

cloning from another Mac over target disk mode, or cloning from an existing Firewire or USB external hard drive.

The Mac must be older than July 25, 2012, and not be a CoreSolo or CoreDuo processor. Core2Duo are not the same, Xeon are not the same can take Lion. G5 and older processors don't take it either.