dfi: 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
/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/lib/seqan/bin/dfi - from package: seqan-apps

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