a5toa4: 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/a5toa4 - from package: texlive-extra-utils

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

Error in other languages:
a5toa4: bevel nie gevind nie
a5toa4: comandă negăsită
a5toa4: comando não encontrado
a5toa4: commande introuvable
a5toa4: command not found
a5toa4: käsku ei ole
a5toa4: không tìm thấy lệnh
a5toa4: komanda nerasta
a5toa4: Kommando nicht gefunden.
a5toa4: kommandot finns inte
a5toa4: komut yok
a5toa4: nie znaleziono polecenia
a5toa4: níor aimsíodh an t-ordú
a5toa4: no se encontró la orden
a5toa4: no s'ha trobat l'ordre
a5toa4: opdracht niet gevonden
a5toa4: parancs nem található
a5toa4: perintah tidak ditemukan
a5toa4: príkaz nenájdený
a5toa4: příkaz nenalezen
a5toa4: команда не найдена
a5toa4: командата не е открита
a5toa4: 命令找不到
a5toa4: コマンドが見つかりません