sx: 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/sx - from package: lrzsz

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

On Debian possible default full paths are:

/usr/bin/sx - from package: lrzsz

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

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