sfscandiff: 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 CentOS possible default full paths are:

/usr/bin/sfscandiff - from package: skipfish

You may install the required package with command yum install {package}

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