nsca-ng: 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 Debian possible default full paths are:

/usr/sbin/nsca-ng - from package: nsca-ng-server

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

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