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="https://stackoverflow.com/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="https://stackoverflow.com/manifest.json">

and remove it from the code to stop the error.

Reference Web App Manifest