that means that URL is already used on some other device to download the same book
Thanks for your answer but:
The problem has arisen since, at the recommendation from the Apple store, I have reverted to OS 10.9 from Yosemite because my laptop is an older one and was running slower. Since the reformat I have reinstalled applications including Adobe Digital Editions. The url provided by Kobo was used on this laptop to install the book previously.