captcha.sh: 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/lib/x86_64-linux-gnu/ejabberd/priv/bin/captcha.sh - from package: ejabberd

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

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