postw90.x: command not found

Have You tried accessing the binary with the full path?

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/postw90.x - from package: wannier90

You may install the required package with command yum install {package}
Error in other languages:
postw90.x: bevel nie gevind nie
postw90.x: comandă negăsită
postw90.x: comando não encontrado
postw90.x: commande introuvable
postw90.x: command not found
postw90.x: käsku ei ole
postw90.x: không tìm thấy lệnh
postw90.x: komanda nerasta
postw90.x: Kommando nicht gefunden.
postw90.x: kommandot finns inte
postw90.x: komut yok
postw90.x: nie znaleziono polecenia
postw90.x: níor aimsíodh an t-ordú
postw90.x: no se encontró la orden
postw90.x: no s'ha trobat l'ordre
postw90.x: opdracht niet gevonden
postw90.x: parancs nem található
postw90.x: perintah tidak ditemukan
postw90.x: príkaz nenájdený
postw90.x: příkaz nenalezen
postw90.x: команда не найдена
postw90.x: командата не е открита
postw90.x: 命令找不到
postw90.x: コマンドが見つかりません