greenbone-nvt-sync: 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/sbin/greenbone-nvt-sync - from package: openvas-scanner

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

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