stunnel: 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 CentOS possible default full paths are:

/usr/bin/stunnel - from package: stunnel

You may install the required package with command yum install {package}

On Debian possible default full paths are:

/usr/bin/stunnel - from package: stunnel4

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

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