These are the instructions recommended by WD staff:
I understand that you have tried the solution below but still not working :
My Cloud Home: macOS Time Machine Error “Backup Disk Not Available”
macOS Time Machine Error "Backup Disk Not Available" on My Cloud Home
Please follow the instructions in the links below:
MY CLOUD OS 5: MACOS TIME MACHINE BACKUP TO A MY CLOUD
My Cloud OS 5: macOS Time Machine Backup to a My Cloud
TIME MACHINE BACKUP TAKES A LONG TIME TO COMPLETE ON NETWORK STORAGE DEVICE
macOS: Time Machine Backup Takes a Long Time to Complete on Network Storage Device
INITIAL TIME MACHINE BACKUP FAILED OR COULD NOT COMPLETE
Why Initial macOS Time Machine Backup Fail?
MACOS TIME MACHINE ONLINE USER GUIDE AND SOLUTIONS
macOS Time Machine: Online User Guide and Solutions
Maybe they are useful for you. My backups are still not working.
