A file can fail to process once it's been uploaded for many reasons, the most common of which is that the file is corrupted, or more rarely it is a format that we cannot transcode.
To check that a file isn't corrupted, simply try opening it. If you can view your content without any issues, then try uploading again make sure you have a stable connection.
If the problem persists, it might be a format that Emortal doesn't support.
For more information on the file formats that we do support, see the FAQ titled: Which file types does Emortal support?