You cannot login because you need to _join_ the community.  
You cannot participate on Area 51 if you didn't **joined Area 51**.  
Creating an account on Stack Exchange does not create accounts for all Stack Exchange sites.  
But when you join the community (or site), your profile will be copied over to the site and your profile will be linked to your Stack Exchange account.  
You need to click <kbd>Sign Up</kbd> or <kbd>Join this community</kbd> then try  to log in.  

<sup><strike>Note: Area 51 signup or join is broken according to [this](https://meta.stackexchange.com/questions/323745/is-area51-login-broken) question.</strike> Area 51 signup is fixed</sup>