[Theme: Creativix] featured image in portfolio doesn't work

I’ve read something about this theme’s portfolio function here
https://www.ads-software.com/support/topic/theme-wp-creativix-featured-picture?replies=7. That you need a WordPress Upload Tool to upload the images you want to feature in the portfolio page template. Unfortunately the author of the message hasn’t explained how.

Why does the usual ‘featured image’ function not work? What is the WordPress Upload Tool and can I do something without using custom code (because I’m not a good coder at all unfortunately).