neb.x: 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/neb.x - from package: quantum-espresso

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

On Debian possible default full paths are:

/usr/bin/neb.x - from package: quantum-espresso

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

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