fitsdiff: 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/fitsdiff - from package: pyfits-utils

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

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