Thumbnails on my home page and other pages aren’t appearing.

What is the solution to this issue? Link to my website: www.theorangebox.studio Problem: 1) Home Page - Thumbnails missing. How to add thumbnails to my recent posts? 2) Project Page, Featured Page, Testimonials Page - Thumbnails missing. How to add thumbnails to these posts? 3) Services Page - Thumbnails missing. How to add thumbnails?