In Ruby on Rails, I added the following code to display error messages for Devise login processing.

■ app/views/devise/sessions/new.html.haml

  -if alert
    #error_explanation
      %h2 Please check messages below:
      %ul
        %li=alert

That’s all.

【Reference】

・Rails - Devise - Error messages when signing in? - Stack Overflow

That’s all from the Gemba.