After a database detach upgrade from 2007 to 2010, you try to do a visual upgrade and get this message:
It is looking for a masterpage that isn't in the 2010 library.
In short, add it to the library. To do this:
1. Go to the upgraded 2010 site, and make a subsite
2. Download a copy of the v4.master page. (Site Settings, Galleries, Master Pages)
3. Go to the upper level (or any site where the error is) and upload the copy of the v4.master into the master page gallery. (Site Settings, Galleries, Master Pages)
4. Try the visual upgrade again.