Reading package lists... Done
Building dependency tree
Reading state information... Done
E: Unable to locate package gcc-6
E: Unable to locate package g++-6
E: Couldn't find any package by regex 'g++-6'
Reading package lists... Done
Building dependency tree
Reading state information... Done
Package ffmpeg 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 'ffmpeg' has no installation candidate