aggregate-avatars: 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/aggregate-avatars - from package: gnome-shell-mailnag

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

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