Add note about ANCM 500.38 error (#17930)

Author: analogrelay

aspnetcore/test/troubleshoot-azure-iis.md

@@ -154,6 +154,15 @@ ANCM failed to start within the provied startup time limit. By default, the time
 
 This error can occur when starting a large number of apps on the same machine. Check for CPU/Memory usage spikes on the server during startup. You may need to stagger the startup process of multiple apps.
 
+### 500.38 ANCM Application DLL Not Found
+
+ANCM failed to locate the application DLL, which should be next to the executable.
+
+This error occurs when hosting an app packaged as a [single-file executable](/dotnet/core/whats-new/dotnet-core-3-0#single-file-executables) using the in-process hosting model. The in-process model requires that the ANCM load the .NET Core app into the existing IIS process. This scenario isn't supported by the single-file deployment model. Use **one** of the following approaches in the app's project file to fix this error:
+
+1. Disable single-file publishing by setting the `PublishSingleFile` MSBuild property to `false`.
+1. Switch to the out-of-process hosting model by setting the `AspNetCoreHostingModel` MSBuild property to `OutOfProcess`.
+
 ### 502.5 Process Failure
 
 The worker process fails. The app doesn't start.