md5sum: 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/md5sum - from package: coreutils

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

On Debian possible default full paths are:

/usr/bin/md5sum - from package: coreutils

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

Error in other languages:
md5sum: bevel nie gevind nie
md5sum: comandă negăsită
md5sum: comando não encontrado
md5sum: commande introuvable
md5sum: command not found
md5sum: käsku ei ole
md5sum: không tìm thấy lệnh
md5sum: komanda nerasta
md5sum: Kommando nicht gefunden.
md5sum: kommandot finns inte
md5sum: komut yok
md5sum: nie znaleziono polecenia
md5sum: níor aimsíodh an t-ordú
md5sum: no se encontró la orden
md5sum: no s'ha trobat l'ordre
md5sum: opdracht niet gevonden
md5sum: parancs nem található
md5sum: perintah tidak ditemukan
md5sum: príkaz nenájdený
md5sum: příkaz nenalezen
md5sum: команда не найдена
md5sum: командата не е открита
md5sum: 命令找不到
md5sum: コマンドが見つかりません