I ran into a Windows Update Server configuration problem a few days ago at a customer’s site. I was were trying to install/configure Windows Update Server on Windows Server 2012. The error was "Fatal Error: Failed to start and configure the WSUS service" while configuring WSUS.
Then I opened the temp log file of WSUS server as shown below. I was getting below marked error i.e. The request failed with HTTP status 503: Service Unavailable.
Actually issue was with the IIS WSUS Administration Website. So in IIS manager console I deleted the "WSUS Administration" website as shown below.
Click Yes to remove selected site.
Now Complete the WSUS installation.