cdfsum: 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/cdfsum - from package: cdftools

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

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