nex: 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/nex - from package: nvi

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

On Debian possible default full paths are:

/usr/bin/nex - from package: nvi

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

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