dm: 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/dm - from package: diffmark

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

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