After typing the command "apt-get install openssh-server", the message below pops up:
Reading package lists... Done
Building dependency tree
Reading state information... Done
Package openssh-server 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 'openssh-server' has no installation candidate
I googled this message for a while and found this quite common when installing softwares.

Is there something not yet done so this problem is here? Any help appreciated.