When I was writing the test code and tried to display the error message, I didn't understand it, so keep a record.
In the view directory
ruby:_error_messages.html.erb
<% if model.errors.any? %>
<div id="error_explanation" class="alert alert-danger">
<ul>
<% model.errors.full_messages.each do |message| %>
<li><%= message %></li>
<% end %>
</ul>
</div>
<% end %>
And call it with the render method.
The model on the first line is supposed to register something in the database this time.
Calling code
<%= render 'error_messages', model: f.object %>
Depending on the location of the file, you need to specify something like shared / error_messages.