At what step of the download process does that happen?
When you click "Download" on a description page, with an error message from it? In the web browser, wirh an error message from it? When you add the downloaded torrent to your torrent client programme, with an error message from it? When it is added to the torrent client programme, but don't download?If there is an error message, what does it say?
Could you please tell as well what torrent client programme you are using (µTorrent/BitTorrent, Vuze/Azureus, Transmission, BitComet, …).
Did you use our site some time ago already?