ztelnet: 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/ztelnet - from package: zssh

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