dfa2dot: 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/dfa2dot - from package: mona

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

Error in other languages:
dfa2dot: bevel nie gevind nie
dfa2dot: comandă negăsită
dfa2dot: comando não encontrado
dfa2dot: commande introuvable
dfa2dot: command not found
dfa2dot: käsku ei ole
dfa2dot: không tìm thấy lệnh
dfa2dot: komanda nerasta
dfa2dot: Kommando nicht gefunden.
dfa2dot: kommandot finns inte
dfa2dot: komut yok
dfa2dot: nie znaleziono polecenia
dfa2dot: níor aimsíodh an t-ordú
dfa2dot: no se encontró la orden
dfa2dot: no s'ha trobat l'ordre
dfa2dot: opdracht niet gevonden
dfa2dot: parancs nem található
dfa2dot: perintah tidak ditemukan
dfa2dot: príkaz nenájdený
dfa2dot: příkaz nenalezen
dfa2dot: команда не найдена
dfa2dot: командата не е открита
dfa2dot: 命令找不到
dfa2dot: コマンドが見つかりません