When you run a site collection health check before upgrading to SharePoint 2013, you may find that the site template is missing for some of the sites within your site collection. There are two reasons this can happen:
The site template is has been deleted.
The site template is not available in SharePoint 2013 for the language in which the site was created.
To fix this issue, you’ll need to do one of the following:
If you’re using SharePoint Server 2013, contact your Farm Administrator.
If you’re using SharePoint Online 2013, contact support.
If the missing site template isn’t available, you will need to back up your data and delete any web sites that use the missing site template and recreate them using an alternative template.