First try clicking Problems? on the overview tab and see if that auto-fixes it.
If you are bringing a SD card from an old phone to the S4, you have to change TB's storage location because of the internal SD card. If "Problems?" doesn't find it for you, go to Menu -> Preferences -> Backup Folder Location: Click DETECT! and change the field to /storage/extSdCard/TitaniumBackup (or whatever folder is on your external SD card)