y4mdenoise: 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/y4mdenoise - from package: mjpegtools

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

Error in other languages:
y4mdenoise: bevel nie gevind nie
y4mdenoise: comandă negăsită
y4mdenoise: comando não encontrado
y4mdenoise: commande introuvable
y4mdenoise: command not found
y4mdenoise: käsku ei ole
y4mdenoise: không tìm thấy lệnh
y4mdenoise: komanda nerasta
y4mdenoise: Kommando nicht gefunden.
y4mdenoise: kommandot finns inte
y4mdenoise: komut yok
y4mdenoise: nie znaleziono polecenia
y4mdenoise: níor aimsíodh an t-ordú
y4mdenoise: no se encontró la orden
y4mdenoise: no s'ha trobat l'ordre
y4mdenoise: opdracht niet gevonden
y4mdenoise: parancs nem található
y4mdenoise: perintah tidak ditemukan
y4mdenoise: príkaz nenájdený
y4mdenoise: příkaz nenalezen
y4mdenoise: команда не найдена
y4mdenoise: командата не е открита
y4mdenoise: 命令找不到
y4mdenoise: コマンドが見つかりません