Windows Admin Center is a web application that makes it easier to manage multiple servers via multiple Microsoft tools such System Center, RSAT, Intune, and PowerShell. If you install Windows Admin Center in an IIS server, you may come across “503 Service Unavailable” error.
Note: This post explains 503 error related to Windows Admin Center only. If the root cause is not this product, check this post out: HTTP 503 Service Unavailable (Application pool has been disabled)