I'm doing rails-tutorial.AWS reports a Blocked host: error.

I am a programming beginner doing rails-tutorial.
As I proceeded with the tutorial, I received a Blocked host: error on the AWS server.
You want to know how to resolve this error.

I will write the details of the situation.
·I am a beginner who started the sixth edition of rails-tutorial.
"·This is an error that occurred when we proceeded with the tutorial in Chapter 1 and moved it to the point where ""rails server"" was executed."
"·In the course of the tutorial, I was instructed to add ""config.hosts.clear"" to ""/hello_app/config/environments/development.rb"", so I followed the instructions."
"·I searched ""rails-tutorial blocked host"" for the error, but I could only find it by adding ""config.hosts.clear"" above."(Or there was a solution that I couldn't understand due to my lack of knowledge, and it was so difficult that I didn't even know what to understand.)
·I thought about other causes, but I couldn't think of it myself, so I asked you a question.

These are the details of the situation and the problems you would like to resolve.
If there is any missing information, I will add it.

I will also write my own prediction below.
I just added config.hosts.clear to the development.rb file, but I didn't do anything like "save?" from there.I just wrote it down, so I thought I needed something like saving it, so I looked it up, but I couldn't search it well.

I have also attached an image.

I would appreciate it if you could reply.Thank you.