Why I cannot find the thank you page option on my form settings?

Why would you suggest a solution when that option for Thank You Page isn't even in the software?

It's possible that your form is a card form, for this layout the thank you page wizard was moved to the very bottom of the builder, here is a guide that will help you finding it: https://www.jotform.com/help/499-How-to-Edit-Thank-You-Page-Settings 

I hope this helps.