stg-mdiff: 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

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/stg-mdiff - from package: stgit-contrib

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