Microsoft maintains a secret, unlisted archive for enterprise users. You can sometimes retrieve the legitimate installer via this direct URL pattern: https://go.microsoft.com/fwlink/?linkid=871743 (Note: This link changes over time and requires a valid license key). Without a key, it’s useless.
: Office 2016 (specifically builds around 16.16 ) is the final release that officially supports El Capitan. Microsoft maintains a secret
Since you're using the last compatible version, you should disable automatic updates for Office. Updating could break compatibility or install a newer, unsupported build. You can do this by opening any Office app (like Word), clicking > Check for Updates , and then unchecking the option to "Automatically keep Microsoft apps up to date." Check for Updates