nicovideo-dl: 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/nicovideo-dl - from package: nicovideo-dl

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

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