'Package name is marked for translation, but there are no valid message files.'

The message files are stored in _locales/[2-letter-locale-abbr]/messsages.json w/ the following example content:

{
  "appName": { "message": "ChatGPT-Breitbildmodus" },
  "appDesc": { "message": "Fügt Widescreen + Full-Window-Modi zu ChatGPT hinzu, um das Scrollen zu reduzieren", }
}

manifest.json contains the following:

    "name": "__MSG_appName__",
    "description": "__MSG_appDesc__",

(Package validates properly in Chrome & Edge store)