Online help needed

I can connect and sign in fine, but when i tried to battle my friend, he said my name didn’t exist?
edit: Is it because i am using the 1.2.3 core? Do I need to do something else to have it work properly and play online?

Both of you being on 1.2.3 core shouldn’t matter, though battling in general is quite bugged.

The best you can try is sending the requests at the same moment, after verifying your in game online account names.