ncdiff: 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/ncdiff - from package: nco

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

On Debian possible default full paths are:

/usr/bin/ncdiff - from package: nco

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

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