when I try to install the command pdfsam:
$ sudo apt-get install pdfsam
I get the following message:
Reading package lists... Done
Building dependency tree       
Reading state information... Done
Package pdfsam 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 'pdfsam' has no installation candidate
It doesn't just do that with this command, but with various. Can anybody tell me what's going on here?