To update the AT&T model of Galaxy S5,
G900A, to
Android 5.1.1, you will need between
6GB- 7GB of free space on your phone's internal memory otherwise the installation will fail. This is because AT&T are, yet again, expanding the system partition in this release with a view to adding more bloatware in a future update. After this update you will have 1.5GB less of phone storage than the normal Samsung phones worldwide. See, also,
Att update to 5.1
Also, the phone will not update if it is rooted, as yours may well be as you are still on the old KitKat firmware, and you also need to be registered on the AT&T network to receive the update.