On share that you choose to create for TFS Backup make sure you add the flowing special account that its named: SERVICE and allocate to this special account Read, Change (or even Full Control) for this share.
Of course, you need to give Read, Change also the user who has administrative rights on TFS installation. (usually the person who installed SQL, TFS)
At this moment, go to your TFS 2013 Administration Console and reconfigure your backup schedule it should be OK and initial check verification should be successful. Of course, you can play more with Security tab of this special share that TFS 2013 will use for storing backups.
Please let you comment if this was helpfully to you.
