if you know their name, type in https://www.codewars.com/users/their-name (= click on their name anywhere it appears) and click Follow
if you don't know the exact name, ask them somewhere else what their name is here, or use a regular search engine with something like site:codewars.com searchterm

I can not see any output results only red and green lights are appeared. Is it happed to all or only me?

How do you follow someone? I can't search for users