.single-post .post-thumbnail { display: none; }

That’s the easiest way, but the page still loads image, it’s just hidden by CSS.

To not output image, we have to create a child theme and then override the template. If you want to go that route, here is the info on childtheme for starter

https://codex.www.ads-software.com/Child_Themes

Once you have your child theme working properly, you can ask question in theme’s support forum so that theme’s author or people who use this same theme can help accordingly.

CleanPortfolio theme support forum
https://www.ads-software.com/support/theme/cleanportfolio