nebula: 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/nebula - from package: nebula

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

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