Neither my iPhone or iPad will back up to the cloud. It says failure? How do I backup to the cloud?

Welcome to the Apple Community.

If your back up continually fails, you might try turning off back up on your mobile device (settings > iCloud > storage & backup) and then deleting the backup file from iCloud by swiping the backup file on the mobile device (settings > iCloud > storage & backup > manage…) and then turning back up on again.

Being able to back up to the cloud can be very useful, especially if you don't have access to a computer or have infrequent access to one, however unless you specifically need to use iCloud for back up, you will find backing up to iTunes significantly more convenient and possibly more reliable.