It won't work if the email application is not Apple Mail, or you are using Apple Mail as a front-end for a third-party email application. If you are directly using Outlook for Mac, Microsoft may have included an Outlook Mail extension sharing component that you would need to enable, but even then, since I don't use Outlook, it may not work with Pages export.
Look in System Settings > General > Login Items & Extensions > Extensions > Sharing to see if there is a vendor-specific item for the third-party mail client, or if Apple Mail is enabled.
If in fact, you are using a third-party email client, and there is no extension to enable it in the Sharing menu, then you will need Export To > PDF > Save… and then drag and drop that resulting saved PDF into the Mail client for attachment.