ssdiff: 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/ssdiff - from package: gnumeric

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

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