viper: 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/viper - from package: python-viper

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