sendsms: 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/sendsms - from package: gnokii

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

On Debian possible default full paths are:

/usr/bin/sendsms - from package: gnokii-cli

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

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