I just switched Carriers from Verizon to MetroPCS. The guy in the store just swapped in a new SIM card in my Galaxy S6. He didn't seem to make any other modifications in my Network Settings or anything. I manually added a Mobile Network for MetroPCS based on another guy's phone (on MPCS).
Anyway, things seem to be working fine (network, WiFi, Data, phone calls). But I now have a strange problem. When I click on SETTINGS / SOFTWARE UPDATE / CHECK FOR SOFTWARE UPDATE, I get an error = "Service Unavailable. Sorry, we are having trouble authenticating your device. Try again later."
Can anyone explain WHY this is happening? My phone is UNLOCKED - albeit purchased 2 years ago at a Verizon store. (It complains that my new SIM card is not a Verizon Sim card. ugh.)
Thanks in advance.
Anyway, things seem to be working fine (network, WiFi, Data, phone calls). But I now have a strange problem. When I click on SETTINGS / SOFTWARE UPDATE / CHECK FOR SOFTWARE UPDATE, I get an error = "Service Unavailable. Sorry, we are having trouble authenticating your device. Try again later."
Can anyone explain WHY this is happening? My phone is UNLOCKED - albeit purchased 2 years ago at a Verizon store. (It complains that my new SIM card is not a Verizon Sim card. ugh.)
Thanks in advance.

maybe he can give you a definitive answer. I looked up that model and it was originally shipped with Android 5.1 and upgradable to 6.01, officially. That's probably where it will stop unless you want to mess with custom roms.