If you see this error message, Office 365 products you’re using might be managed by your organization. Some organizations may block downloads of new add-ins from the Office Store to prevent their users from downloading add-ins that have not been approved by the organization:
You are welcome to check this article for more information on the subject.
To resolve the issue, please contact your Office 365 administrator to ensure that you have permission to install Office Add-ins.
If Grammarly for Microsoft Word on Mac doesn’t work after trying these steps, and you’re using macOS 10.12 or newer, we suggest switching to Grammarly for Mac. This product offering also supports integration with Microsoft Word and Outlook. To find out more, check out this article.