sum: 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/sum - from package: coreutils

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

On Debian possible default full paths are:

/usr/bin/sum - from package: coreutils

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

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