cowbell: command not found

Have You tried accessing the binary with the full path?

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/cowbell - from package: cowbell

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