In conclusion, installing the gem solved it.
What is gem?
There are two.
This time, the purpose is to start rais s, so I won't go into details, but it's a good idea to leave it in your head.
gem install rails
When you type in the terminal,
Successfully installed rails-220.127.116.11 Parsing documentation for rails-18.104.22.168 Done installing documentation for rails after 0 seconds 1 gem installed
Rails installation was successful, It says that gem is also installed!
When I type in, no error message is displayed, http://localhost:3000/ When I open
This is a success! !! First of all, it is good to check whether it is displayed.