Maybe you need to uninstall & reinstall Office, which is the normal way of repairing broken Office apps on Macs... Unless your product key was a pirated one, it should still be accepted. Office 2011 still runs on Macs. Evidently, though, you've unwisely installed a trial copy of Office 365 over that and the trial has now expired. In that case, it's Office 365 you'll need to uninstall, then reinstall Office 2011 - assuming Office 2011 meets your needs; otherwise, take out an Office 365 subscription.

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Cheers,
Paul Edstein
[Fmr MS MVP - Word]