If your Sitemap is not publishing we suspect there may be two main reasons.
1) You don't have a valid SSL Certificate
Make sure that your site has a valid SSL certificate. If your site does not have a SSL certificate, per their policy Shopify blocks access to the page added by our app. Unfortunately this is not due to from our app, but due to Shopify.
2) Make Sure App is Enabled
Please enter the settings menu of the app and click on the enable box and then hit save. You may refer to below image.