Full screen slider not showing

Solved. It didn’t have an image for starters, and now i uploaded one of mine (I imagined it would come with a demo image)

Hi, I’m quite new to Wordpress, but am following an online course on Sydney theme. When customizing the header type (both for front page or site header types) the demo full screen slider doesn’t show up (it shows only white with no header at all, and when hover over the menu the words highlight in red). This happened from the beginning just after activating the theme.
I’m doing as localhost in case it matters.
What am I doing wrong? :confused: