The Office Connector plugin is a System plugin and requires sysadmin permission to be configured and that level of permission is not available on Cloud. You'll find a note at the right hand side of the documentation here: https://confluence.atlassian.com/conf56/confluence-administrator-s-guide/integrating-confluence-with-other-applications/configuring-the-office-connector where it says that it doesn't apply to Confluence Cloud.
If its not permitted, then it needs to be removed from the front-end as well. I have users asking why their "Open in Office" button doesn't work, and its pretty difficult to explain that the Administrator does not have control of the site.
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