This really helped me when I encountered the 'cannot be uploaded, Error message' issue with CyberPanel. I had no root access, so nothing I can do with the configuration. Tried FileZilla, net2ftp but still no luck, I didn't even see the port (21) is blocked. If you have the same issue, give this a try.
Upload (or create the file manually) ftp.php to public_html. Then head to yoursite.com/ftp.phpHope it helps! :)