postmaster: 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/postmaster - from package: postgresql-server

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

On Debian possible default full paths are:

/usr/lib/postgresql/9.4/bin/postmaster - from package: postgresql-9.4

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

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