Won't let me at among us with a account

The reason for this message is a security feature called Sign In with Apple

See this link for more reference about Sign In with Apple

What is Sign in with Apple? - Apple Support

See this link on how to enable Sign In with Apple

How to use Sign in with Apple – Apple Support (UK)

Not all apps accept Sign In with Apple.

then does that mean that the app is not secure

The app can be secure without Sign In with Apple. Not all apps have been rewritten to accept Sign In with Apple.