a+: 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

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/a+ - from package: aplus-fsf

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