plmerge: 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/plmerge - from package: gnustep-base

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

On Debian possible default full paths are:

/usr/bin/plmerge - from package: gnustep-base-runtime

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

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