dnadiff: 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/dnadiff - from package: mummer

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

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