GCP App Engine unable to serve resource files like JavaScript

I have hosted my dotnetcore 2.0 app on GCP App Engine with my app.yaml as follows.

runtime: aspnetcore
env: flex
manual_scaling:
  instances: 1
resources:
  cpu: 1
  memory_gb: 0.5
  disk_size_gb: 10  
endpoints_api_service:
  name:  ***********
  rollout_strategy: managed

My API's work fine, but when I try to load a cshtml view with resources pointing to wwwroot, I get an error loading resources (ex: .js , .css)

{
 "code": 5,
 "message": "Method does not exist.",
 "details": [
  {
   "@type": "type.googleapis.com/google.rpc.DebugInfo",
   "stackEntries": [],
   "detail": "service_control"
  }
 ]
}

My purpose here is to have an dotnetcore app serving web API's as well as an Angular6 UI SPA.