Comment 2 for bug 1874953

This really looks like an environment issue:

Setting up smartmontools (7.1-1build1) ...

Configuration file '/etc/default/smartmontools'
 ==> Modified (by you or by a script) since installation.
 ==> Package distributor has shipped an updated version.
   What would you like to do about it ? Your options are:
    Y or I : install the package maintainer's version
    N or O : keep your currently-installed version
      D : show the differences between the versions
      Z : start a shell to examine the situation
 The default action is to keep your current version.
*** smartmontools (Y/I/N/O/D/Z) [default=N] ? d
sh: 1: pager: not found
diff: standard output: Broken pipe
dpkg: error processing package smartmontools (--configure):
 conffile difference visualizer subprocess returned error exit status 127

Due to missing /bin/pager:

(c)rafaeldtinoco@devel:~$ which pager
(c)rafaeldtinoco@devel:~$ ls -lah /bin/pager
lrwxrwxrwx 1 root root 23 Apr 24 21:39 /bin/pager -> /etc/alternatives/pager
(c)rafaeldtinoco@devel:~$ ls -lah /etc/alternatives/pager
lrwxrwxrwx 1 root root 13 Apr 24 21:39 /etc/alternatives/pager -> /usr/bin/less

Try checking your system for pager:

(c)rafaeldtinoco@devel:~$ sudo update-alternatives --query pager
Name: pager
Link: /usr/bin/pager
Slaves:
 pager.1.gz /usr/share/man/man1/pager.1.gz
Status: auto
Best: /usr/bin/less
Value: /usr/bin/less

Alternative: /bin/more
Priority: 50
Slaves:
 pager.1.gz /usr/share/man/man1/more.1.gz

Alternative: /usr/bin/less
Priority: 77
Slaves:
 pager.1.gz /usr/share/man/man1/less.1.gz

and fixing it!