openvas-nvt-sync-cron: command not found

Have You tried accessing the binary with the full path?

Maybe Your $PATH variable doesn't contain the default paths? echo $PATH should give something like:

[root@server ~]# echo $PATH

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/openvas-nvt-sync-cron - from package: openvas-scanner

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