I don't have access to the thank you page settings

I need to modify the thank you settings but the option doesn't appear in the setting menu. I've tried cloning the form, but the new form has exactly the same (lack of) settings options. If I start a completely new form the option is present in the menu. Does this mean I have to begin a new form from scratch?

The form on your screenshot has the Cards layout. So, you can find the Thank you Page settings directly at the bottom of BUILD tab:

If you need further assistance, please let us know.

Thank you for contacting us. 

thank you Victoria