fprev: 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
/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/mh/fprev - from package: nmh

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