gunicorn-debian: 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/gunicorn-debian - from package: gunicorn

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

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