You need to reboot your WD My Cloud.
Access WD MyCloud through the Web Interface. Mine is accessible at http://192.168.1.131/UI/, but yours might be at a different location.
You can also try on Safari: Preferences → Advanced → Include Bonjour in the Favourites Bar. You should then be able to find Bonjour → WDMyCloud on your favourites bar.
- On the WD My Cloud Web Interface you should then:
- Settings → General → Mac Backups → toggle Time Machine off
- Settings → Utilities → Device Maintenance → Reboot
- wait for your device to reboot …
- Settings → General → Mac Backups → toggle Time Machine on
- Open Time Machine preferences, select the appropriate disk, and try to backup again.
On my system Time Machine usually takes a while to “prepare the backup” before it actually starts backing up again.