E: Package 'mercurial' has no installation candidate. How can i solve this problem?
sudo apt-get install mercurial
Reading package lists... Done
Building dependency tree
Reading state information... Done
Package mercurial is not available, but is referred to by another package.
This may mean that the package is missing, has been obsoleted, oris only available from another source
sudo gedit /etc/apt/sources.list(as the @andol say) then copy/paste that.
gksudo gedit, or, on Kubuntu,
kdesudo gedit) is preferable because it doesn't create configuration problems for the graphical application being run.