A 'path/not_found' error like this indicates that there was nothing found in the connected account at the path specified by the app. In this case, that's specified by the parameter value you're giving to the DownloadAsync method, which you're building from `folder + "/" + file`.
Those actual values are mostly redacted from your screenshot, but I do see that you have "?dl=1" at the end of your `file` value. Unless you actually have at the end of the filename of the file in your account, you shouldn't be appending that. That's just an option for use with shared links, not for file paths themselves.
Also, note that if your app is registered for the "app folder" permission, all file paths will automatically be interpreted relative to the special app folder automatically made for your app, inside "/Apps" by default. In that case, make sure the file(s) you're trying to work with are actually inside that app folder.
You can use the ListFolderAsync/ListFolderContinueAsync methods to list the contents, if any, of the root folder, by starting with a 'path' value of the empty string "". That way you can get the actual Metadata.PathLower value for downloading a file with DownloadAsync.