I have samsung 8 and it worked in the beginning. I have freinds accounts and it does the same when I try with their account. I have uninstalled pokemon go and reinstalled it still same problem. HELP
Sounds like a server issue on Niantic's end or maybe an issue with your account. I would contact Niantic support and explain the issue to them and they can check into it.
I have seen that happen to someone. We ended up all backing out and then the player closed out all apps then rebooted phone and was able to get back in.