For example, after changing the Tsinghua source and installing 'libgtk2.0-dev', the following appears:
Package libgtk2.0-dev is not available, but is referred to by another package.
This may mean that the package is missing, has been obsoleted, or
is only available from another source
E: Package 'libgtk2.0-dev' has no installation candidate
Solution:
reload
and then execute:
sudo apt-get update
Just install it again
sudo apt-get install libgtk2.0-dev