The add-in hasn't been approved

Check in the App Catalog if the Leave Calendar add-in is approved. Go to https://***-admin.sharepoint.com/_layouts/15/online/ManageAppCatalog.aspx (where *** is the name of your SharePoint domain). If you don't have an app catalog setup, approval for apps isn't possible and the only way to add the add-in is from Purchase an App in SharePoint Admin Center. See the installation guide on this page for full instructions.n)