You click Option and then open the Share window to share a resource for a SharePoint Online site, folder or file. When you view the Shared with tab, you notice that the EMAIL EVERYONE option is missing.
The EMAIL EVERYONE option is available only when the number of users that the SharePoint Online site, folder, or file is shared with is less than 30. If the site, folder, or file is already shared with more than 30 users, you cannot use the EMAIL EVERYONE functionality.
This is expected behavior in SharePoint Online.