azure static web apps fail to build simple html web page: Could not detect the language from repo

my devops git project looks like this:

index.html
my.js
my.css

The pipeline then gives this error:

Detecting platforms...
Could not detect any platform in the source directory.
Error: Could not detect the language from repo.


---End of Oryx build logs---
Oryx was unable to determine the build steps. Continuing assuming the assets in this folder are already built. If this is an unexpected behavior please contact support.
An unknown exception has occurred

For further information, please visit the Azure Static Web Apps documentation at https://docs.microsoft.com/en-us/azure/static-web-apps/
If you believe this behavior is unexpected, please raise a GitHub issue at https://github.com/azure/static-web-apps/issues/
Exiting
##[error]Error: The process '/usr/bin/bash' failed with exit code 1
Finishing: AzureStaticWebApp

Azure added this build file:

name: Azure Static Web Apps CI/CD

pr:
  branches:
    include:
      - main
trigger:
  branches:
    include:
      - main

jobs:
- job: build_and_deploy_job
  displayName: Build and Deploy Job
  condition: or(eq(variables['Build.Reason'], 'Manual'),or(eq(variables['Build.Reason'], 'PullRequest'),eq(variables['Build.Reason'], 'IndividualCI')))
  pool:
    vmImage: ubuntu-latest
  variables:
  - group: Azure-Static-Web-Apps-xxx-variable-group
  steps:
  - checkout: self
    submodules: true
  - task: AzureStaticWebApp@0
    inputs:
      azure_static_web_apps_api_token: $(AZURE_STATIC_WEB_APPS_API_TOKEN_xxx)
###### Repository/Build Configurations - These values can be configured to match your app requirements. ######
# For more information regarding Static Web App workflow configurations, please visit: https://aka.ms/swaworkflowconfig
      app_location: "/" # App source code path
      api_location: "" # Api source code path - optional
      output_location: "/" # Built app content directory - optional
###### End of Repository/Build Configurations ######

I also tried it with this line:

  output_location: "" 

but get same error.

In the "application settings" in azure static web app "configuration" page, I tried the following:

  ENABLE_ORYX_BUILD = false

This got a bit further, but still failed.

tried adding this:

inputs:
  skip_app_build: true

Which gives this error:

 App Directory Location: '/' was found.
 Looking for event info
 Skipping step to build /working_dir with Oryx
 An unknown exception has occurred

==== update ====
Also tried adding a dummy package.json, in case there is no way to avoid azure trying to use npm. using this package.json in the root:

{
  "name": "xxxx",
  "version": "0.0.1",
  "description": "xxxx",
  "main": "index.js",
  "repository": "git@ssh.dev.azure.com:v3/xxxx",
  "author": "x@y.com",
  "license": "MIT",
  "private": true,
  "scripts": {
    "start": "npx live-server"
  }
}

gave this error:

Detecting platforms...
Detected following platforms:
  nodejs: 16.19.0
Version '16.19.0' of platform 'nodejs' is not installed. Generating script to install it...
Error: Could not find either 'build' or 'build:azure' node under 'scripts' in package.json. Could not find value for custom run build command using the environment variable key 'RUN_BUILD_COMMAND'.Could not find tools for building monorepos, no 'lerna.json' or 'lage.config.js' files found.

I tried it again today, and its now building. The issue was either

1. azure build system was broken for some days
2. I deleted one file in the project which was a symbolic link which should not have been there.