aa: 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/aa - from package: astronomical-almanac

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

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