syslog-ng: 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:

/sbin/syslog-ng - from package: syslog-ng

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

On Debian possible default full paths are:

/usr/sbin/syslog-ng - from package: syslog-ng-core

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

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