stag-diff: 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/stag-diff - from package: libdata-stag-perl

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

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