pgrep: 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/pgrep - from package: procps

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

On Debian possible default full paths are:

/usr/bin/pgrep - from package: procps

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

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