tl; dr
--I started the Rails server for the first time, but I can't access localhost: 3000 first.
--It seems that the personal firewall of ESET (security software) is doing something wrong.
--I was able to access with 127.0.0.1:3000 instead of localhost: 3000
[Why can't I connect to a web application under development with [:: 1] on a mac with eset?] (https://ja.stackoverflow.com/questions/66740/eset-%E3%82%92%E5%85%A5%E3%82%8C%E3%81%9F-mac-%E3%81%A7-1-%E3%81%A7%E9%96%8B%E7%99%BA%E4%B8%AD%E3%82%A6%E3%82%A7%E3%83%96%E3%82%A2%E3%83%97%E3%83%AA%E3%82%B1%E3%83%BC%E3%82%B7%E3%83%A7%E3%83%B3%E3%81%AB%E6%8E%A5%E7%B6%9A%E3%81%A7%E3%81%8D%E3%81%AA%E3%81%84%E3%81%AE%E3%81%AF%E4%BD%95%E6%95%85)
It seems that it cannot be accessed via IPv6 and can be accessed via IPv4. I'm not sure what the IP address is or why it was resolved, so I'd like to read a book.