Return {"path/not_found/..."}

HI All 

I have an issue in C#  dbx.Files.GetTemporaryLinkAsync method

when I pass a root folder it generates the URL and works like a charm but when you send the sub-folder path it returns 

{"path/not_found/..."}

Any idea, please 

 

public static string GetTempDropboxImageUrl(string File,string token)
{
var dbx = new DropboxClient(token);
var response = dbx.Files.GetTemporaryLinkAsync(File).Result;
return response.Link;
}