Hello!
I just tested and checked out, everything seems fine.
From the screenshot you provided, it looks like you entered your username with "@". Make sure you enter your TikTok username without the "@" symbol, as explained in the game and in the "Help Guide & Tips" document provided. This specific message isn’t 100% accurate (it’s a known bug with TikTok servers that I’m trying to resolve).
If you have any further questions or need additional assistance, feel free to contact me directly at livegameyouplay@gmail.com.