How to edit thank you message in new form layout?

I am trying to customize my Thank You page, however there is no Thank You page listed on the left bar when I go to Settings.

Please let me know how I can get this to show up so I can then customize it.

Thank you!

I'm sorry for the trouble caused to you. 

I did check your last edited form and it seems you are using new form layout. You should be able to edit the thank you message of your form by clicking on the "Edit Thank You Page" which is the last element of your form. Please check the screenshot below on how to edit thank you message in new form layout:

Hope this helps.

Do get back to us if you have any questions.