barnowl: 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/barnowl - from package: barnowl

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

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