The older versions of Office installer have a certificate within them to make sure they are not tampered with. This certificate has an expiration date which on the 2011 version has expired. I would recommend you get a newer version that works on your given OS-X/macOS or you’ll need to alter your systems date back to 2012/13 so the installer thinks its back within the certificates window.
I would also make sure you are running the highest version of OS-X/macOS your system supports as well to be safe.
For reference: [https://appleinsider.com/articles/19/10/24/if-youve-got-an-old-macos-install-image-it-will-probably-stop-working-today|If you've got an old macOS install image, it will probably stop working today]