Do you know that you need to migrate 'custom site template' before upgrade to SharePoint 2013 UI?
Updated by Brady Stroud [SSW] 1 year ago. See history
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If you have “custom site template” for your site, you can’t upgrade your site to the SharePoint 2013 UI unless you have a site template with the same name ready for new UI.  Figure:SharePoint will show you an error “Missing Site Templates” that prevents you from upgrading To fix this issue 1. Upgrade your site template’s **content** files and **definition** XML file to SharePoint 2013 (refer to SharePoint 2013 default site template for details). 2. Package the site template’s **content** files to map location “ **{SharePointRoot}\Template\SiteTemplate** ”. >  > > 3.Package the site template’s **definition** XML file to map location “ **{SharePointRoot}\TEMPLATE\1033\XML** ”. >  > 4.Deploy the package. > 5.Try to upgrade to SharePoint 2013 UI again.