telnet-client: 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 CentOS possible default full paths are:

/usr/bin/telnet-client - from package: libtelnet-utils

You may install the required package with command yum install {package}

On Debian possible default full paths are:

/usr/bin/telnet-client - from package: libtelnet-utils

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

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