Hi @Manasa Gowda ,
Per my test, unfortunately, I can not reproduce your issue. The code works well in my test. As a troubleshooting, please try to delete var NavigationTitle = webAskAdd.Navigation.TopNavigationBar.Add(node);
in your code and run it again.
More information for reference:
https://www.codesharepoint.com/csom/create-sub-site-in-sharepoint-using-csom
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