This page loads an archive page for Testimonials post type. Since you don’t have any testimonial entry yet in the Dashboard > Testimonials, you’ll get a “Nothing Found” page. This is not an issue to resolve. WordPress system works that way to recognize custom post type archive page.
The solution is to create a new page and name it as “Our Testimonials” or something else you prefer. So the page URL will be different and will look like yoursite.com/our-testimonials.
Besides, what do you mean by flush the permalink?
The common reason why a page returns a “Nothing Found” is due to permalink configuration in the .htaccess file can’t be updated automatically. The first thing to do to resolve this is to flush the permalink. But, in your case, it doesn’t relate with this. You’ll only need to use different page slug URL by composing a new page.
To help you easier to understand, please read my reply on other old topic here.