The following sites are using
templates that are not yet supported after upgrade (or)
Sites that rely on these templates cannot be upgraded at this time, preventing
upgrade of this site collection
Issue: when we upgrade any site from SharePoint 2010 to SharePoint
2013 and that site contains custom site definitions, we face the issue.
Solution: Ensure to follow the steps below to resolve the
issue.
1. Both versions of the site definitions (14 and 15) need to be present
in respective hives at the time of performing upgrade. If either of them
is not there then the above error will show up.
2. Uninstall > Remove and Add > Install. This had removed the
previous site definition files (one and template folder) from 14 hive and added
latest files to 15 hive.
I hope the above information will help you to resolve the issue, in case of any queries/questions regarding the above mentioned information then please let me know. I would be more than happy to help you as well as resolves your issues, Thank you.
