pmdiff: 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/pmdiff - from package: pcp

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

On Debian possible default full paths are:

/usr/bin/pmdiff - from package: pcp

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

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