smssend: 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/smssend - from package: smstools

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

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