No app can be installed in my system after I recently upgraded from Ubuntu 19.10 to 20.04.
$ sudo apt-get install vlc gimp gparted synaptic
Reading package lists... Done
Building dependency tree
Reading state information... Done
Package synaptic 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
Package gimp 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
Package vlc 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 'vlc' has no installation candidate
E: Package 'gimp' has no installation candidate
E: Package 'synaptic' has no installation candidate
How can I fix this issue?