Q:

HTTP Error 503. The service is unavailable in IIS?

I published my Web Api (framework 4.5) in this path C:\inetpub\wwwroot\MyWebAPI
In my IIS i converted this folder to an app
When i try to access this URL http://localhost/MyWebAPI/api/client/1 a message appears
Service Unavailable
HTTP Error 503. The service is unavailable.
Why?

My App Pool is set to be "STOP", i turn it on but it goes back to "STOP"

A:

I found the solution

Click on Application Pools under the tree with your machine name, 
on the right side, click click on Advanced Settings,
in Process Model change the "Load User Profile" to false
Start your apllication and restart your IIS

The error will surelly change, in my case the error changed to this one

"Config Error: This configuration section cannot be used at this path.
  This happens when the section is locked at a parent level. Locking is
  either by default (overrideModeDefault="Deny"), or set explicitly by a
  location tag with overrideMode="Deny" or the legacy
  allowOverride="false"."

I have found the solution for this other problem here 
IIS - this configuration section cannot be used at this path (configuration locking?)

