Openssh server is not available

[John Doe]

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Hello - when I try to sudo apt-get install openssh-server I get -
"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."  What should I do?  Thanks.
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