How to fix the “Unable to calculate upgrade” issue from command line when upgrading from 12.04 to 14.04?
Solution 1:
Summary of long discussion in comments:
do-release-upgrade failed to run because the locales package was not present. So the solution was to install locales, which is a required package.
The OP handled a dependency problem when attempting to install locales via apt-get install by using aptitude instead, which suggested solutions rather than just refusing to install.