It says some files were unavailable during last back up. What to do?

Hello razib8434,

Thanks for reaching out to the Apple Support Communities! It sounds like you are having issues doing a backup backup to either iCloud or iTunes but you are getting an error about files being unavailable. I understand the need to have backups working as expected. I will be more than glad to help you.
I want to share with you some helpful recourses that might help you:

If iCloud Backup couldn’t be completed or you can’t restore

or

If your iTunes backup couldn‘t be completed or you can‘t restore from a backup

You might need to delete the current backup you just made and try to create a new backup. The next article gives you instructions to accomplish this as well:
iCloud: Remove an iOS device backup from iCloud
Hope this information is of help to you. If you need additional assistance, just let us know.
Cheers!

Soultion should be simple - I just solved this problem on my iPhone.

Go to your existing backups and delete the backup of that particular phone. The retry the backup, it should work. Cheers!