stunnel3: 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/stunnel3 - from package: stunnel4

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

Error in other languages:
stunnel3: bevel nie gevind nie
stunnel3: comandă negăsită
stunnel3: comando não encontrado
stunnel3: commande introuvable
stunnel3: command not found
stunnel3: käsku ei ole
stunnel3: không tìm thấy lệnh
stunnel3: komanda nerasta
stunnel3: Kommando nicht gefunden.
stunnel3: kommandot finns inte
stunnel3: komut yok
stunnel3: nie znaleziono polecenia
stunnel3: níor aimsíodh an t-ordú
stunnel3: no se encontró la orden
stunnel3: no s'ha trobat l'ordre
stunnel3: opdracht niet gevonden
stunnel3: parancs nem található
stunnel3: perintah tidak ditemukan
stunnel3: príkaz nenájdený
stunnel3: příkaz nenalezen
stunnel3: команда не найдена
stunnel3: командата не е открита
stunnel3: 命令找不到
stunnel3: コマンドが見つかりません