Same for me. I entered my birthday date but I can't change quick chat for free chat even if I'm 43 years old, and I get the same error each time I try to sign in.
Sorry, it still doesn't work, even with the last version of Among Us. Now I can use PlayerPrefs to choose a name instead of getting a random one, but I'm still unable to sign in (errror: PlatformNotSupported while I'm on PC) and to use free chat, even if I change dates. I'll try to contact Innersloth support. Thanks for trying to help me.