sfddiff: 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/sfddiff - from package: fontforge

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

On Debian possible default full paths are:

/usr/bin/sfddiff - from package: fontforge-nox
/usr/bin/sfddiff - from package: fontforge

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

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