How are you trying to download it?
Which Kobo? I think the wireless ereaders can download Kobo books directly, but need to go via ADE and usb connection for other books.
And the tablets are different again.
If you are using Digital Editions (ADE) , then you need meny/Help/Authorize Computer and it will prompt you.
Library/Authorize Computer on the older more reliable v1.7.2.
Then with the kobo plugged in, meny/Help/Authorize Device (Library/Authorize Device on 1.7.2)