Can't get rid of missing manifest.json error

Most probably there is a reference to manifest.json somewhere in the project, while the file/resource itself does not exist.

Check to see if you have any link tags with rel=manifest similar to

<link rel="manifest" href="/manifest.webmanifest">

The .webmanifest extension is specified in the Media type registration section of the specification, but browsers generally support manifests with other appropriate extensions like .json.

ie

<link rel="manifest" href="/manifest.json">

and remove it from the code to stop the error.

Reference Web App Manifest

The manifest.json file is likely where it's supposed to be. The solution is to add an entry in your web.config file under the static content section for .json files.

<?xml version="1.0" encoding="utf-8" ?>
<configuration>
  <system.webServer>
    <staticContent>
      <mimeMap fileExtension=".json" mimeType="application/json" />
  </staticContent>
</system.webServer>
</configuration>

If you need to add or edit your web.config file, you can do so using the Kudu debug console. (Replace yourapp with your app in the link)

You can also launch the debug console from the portal under Development Tools for your app:

If the manifest.json file is actually missing, you can fix that by following Google's instructions for adding a manifest.json file.

The Web App Manifest is required by Chrome to enable the Add to Home Screen prompt in a web app.

just add crossorigin="use-credentials" so it will look like: <link rel="manifest" href="/site.webmanifest" crossorigin="use-credentials">