libfm-pref-apps: command not found

Have You tried accessing the binary with the full path?

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

[root@server ~]# echo $PATH

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/libfm-pref-apps - from package: libfm-tools

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