postman: 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/postman - from package: python-postman

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

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