Microsoft have taken down the activation servers for old versions of Office. You cannot reactivate even with a valid key. Been there, encountered that. Whoever advised a erase/clean install did you no favors.
Do you have a system backup made before the erasure?
There are no current versions of Office that will install on a 2008 Mac running OS 10.11. Microsoft only supports the current OS version and the previous two.
You may be able to find an older version of the open-source LibreOffice that runs on 10.11 but even that is a long shot today.