Hey @opopop5232! Welcome to the forums!
Unfortunately, if you want to use the Metahuman Core features, you have to upgrade to 5.6, as 5.6 is the version in which it was introduced as part of Unreal Engine. As for the online metahuman creator, I think they took it down, I can’t get to it either.
This isn’t Epic Support, this is a community forum, but I can give you the link to Epic Support and see if they can help you! 
https://www.epicgames.com/help/en-US/