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
.webmanifestextension 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