The action 'index' could not be found for SignupsController

Link to my project:

I have followed the steps and video for guide. When I type in the email and click submit button the page directed to /signups with error message
Suppose should directed to /thanks

SignupsController Action below:

my routes:

I have google it to find a solution for this, but not relevant as per the guide from the help video. Though I give a try by adding index.html.erb under /app/views/signups/ but its not helped me to create email in my db table.
This is the URL suggested to add index.html.erb:

Please highlight my mistakes if there is any and your suggestion will be very helpful. Thanks.