Samsung Galaxy S5 SM-G900A aka AT&T Samsung Galaxy S5 4G LTE, Samsung Pacific
Like many other AT&T device versions, the bootloader of SM-G900A is completely locked down so you cann’t flash a custom recovery like Team Win TWRP.
Have you asked the provider/carrier AT&T for an activation code for this device? I’m afraid the chances aren’t going to be good.
→ The same problem exists with the Samsung Galaxy S5 G900V (verizon)