What version of the OS are you using ?
To find out what is really wrong, open the console.app from applications/utilities and check the log.
You can filter the log for any event that starts with " com.apple.backupd " ( without the quotes ).
That will give you an accurate indication of what is wrong.
Shop for your Apple, Mac, iPhone and other computer products on Amazon.
We are a participant in the Amazon Services LLC Associates Program, an affiliate program designed to provide a means for us to earn fees by linking to Amazon and affiliated sites.