openvas-client: 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/openvas-client - from package: openvas-client

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

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