dm-merge: command not found

Have You tried accessing the binary with the full path?

Maybe Your $PATH variable doesn't contain the default paths? echo $PATH should give something like:

[root@server ~]# echo $PATH

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-merge - from package: diffmark

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