I have bought a Kobo e-book and downloaded it on my computer, but I cannot copy it to my Kobo e-reader. I get the message: CE_COPY_NOT_ALLOWED - No permission to copy the book.

I have bought a Kobo e-book and downloaded it to my computer, but I cannot copy it to my Kobo e-reader. I get the message: CE_COPY_NOT_ALLOWED  - No permission to copy the book. I searched on the internet a got a solution to that specific error message, and I followed that, but to no avail, I still cannot copy my book! I have tried in vain many times. I have previously copied many books. 

What am I going to do?