sms_client: 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/sms_client - from package: smsclient

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