lrfviewer: command not found

Maybe Your $PATH variable doesn't contain the default paths? echo $PATH should give something like:

[root@server ~]# echo $PATH
/usr/local/sbin:/usr/local/bin:/sbin:/bin:/usr/sbin:/usr/bin:/root/bin

If it's empty, You may set the PATH variable with this command

export PATH="/usr/local/sbin:/usr/local/bin:/sbin:/bin:/usr/sbin:/usr/bin:/root/bin"

On Debian possible default full paths are:

/usr/bin/lrfviewer - from package: calibre

You may install the required package with command apt-get install {package}

Error in other languages:
lrfviewer: bevel nie gevind nie
lrfviewer: comandă negăsită
lrfviewer: comando não encontrado
lrfviewer: commande introuvable
lrfviewer: command not found
lrfviewer: käsku ei ole
lrfviewer: không tìm thấy lệnh
lrfviewer: komanda nerasta
lrfviewer: Kommando nicht gefunden.
lrfviewer: kommandot finns inte
lrfviewer: komut yok
lrfviewer: nie znaleziono polecenia
lrfviewer: níor aimsíodh an t-ordú
lrfviewer: no se encontró la orden
lrfviewer: no s'ha trobat l'ordre
lrfviewer: opdracht niet gevonden
lrfviewer: parancs nem található
lrfviewer: perintah tidak ditemukan
lrfviewer: príkaz nenájdený
lrfviewer: příkaz nenalezen
lrfviewer: команда не найдена
lrfviewer: командата не е открита
lrfviewer: 命令找不到
lrfviewer: コマンドが見つかりません