apt-get 설치가 작동하지 않습니다

기계에 표시되는 내용은 다음과 같습니다.

#apt-get install tor
Reading package lists... Done
Building dependency tree       
Reading state information... Done
Package tor 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 'tor' has no installation candidate