notify-send: 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/notify-send - from package: libnotify

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

On Debian possible default full paths are:

/usr/bin/notify-send - from package: libnotify-bin

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

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