time machine couldn't complete backup

I have a new iMac, and an older Western Digital Sharespace network backup. I have the mac and NAS attached to the same switch (the router is full). Time Machine will connect to WD_backup, and I use wd_backup as my user ID ,and backup as my password as indicated. It connects, but no matter what I do I constantly get the same error message.

"Time Machine couldn't complete the backup to "WDSharespace""

"The backup disk is not available"

I am at a complete loss as to why this will not work. Can anyone help?

Thanks