nncheck: 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/bin/nncheck - from package: nn

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

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