stunnel4: 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/stunnel4 - from package: stunnel4

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

Error in other languages:
stunnel4: bevel nie gevind nie
stunnel4: comandă negăsită
stunnel4: comando não encontrado
stunnel4: commande introuvable
stunnel4: command not found
stunnel4: käsku ei ole
stunnel4: không tìm thấy lệnh
stunnel4: komanda nerasta
stunnel4: Kommando nicht gefunden.
stunnel4: kommandot finns inte
stunnel4: komut yok
stunnel4: nie znaleziono polecenia
stunnel4: níor aimsíodh an t-ordú
stunnel4: no se encontró la orden
stunnel4: no s'ha trobat l'ordre
stunnel4: opdracht niet gevonden
stunnel4: parancs nem található
stunnel4: perintah tidak ditemukan
stunnel4: príkaz nenájdený
stunnel4: příkaz nenalezen
stunnel4: команда не найдена
stunnel4: командата не е открита
stunnel4: 命令找不到
stunnel4: コマンドが見つかりません