A brief electrical outage interrupted my 12.04 upgrade. After many tries, which included trying to upgrade from a command line, I was able to access Ubuntu screens again.
I find that the 12.04 upgrade was indeed not completed; I am still running 11.10. However, Update Manager does not give me the option of installing 12.04 now.
And, when I run Update Manager, I am asked if I want to execute a "partial install." When I answer "yes," I receive the message above. Besides a clean install, any ideas about what can I do to upgrade to 12.04?