This looks more like an account/login issue on ManyChat’s side than something you did wrong.
If it keeps saying “Access denied” a short time after creating the account, I’d first try signing in with the same method you used when creating it and make sure you’re not mixing Google / Facebook / email login by accident. ManyChat’s login help page mentions that using the wrong sign-in option can cause access problems.
But if it keeps happening on new accounts again and again, that’s probably not something the community can fix. At that point I’d contact ManyChat Support directly under Account Access & Login and let them check why those accounts are being denied.
So basically: try the exact same login method first, but if it still happens, this is one for support.