My Figma element has transparent background but I want to get it solid, simple as that, and I can't find the correct way how to do it.
Which thing in the settings I should check?
To do what you want:
First: make sure the element fill is 100%, when you are selecting the element layer itself, it's here:
Second: make sure the frame itself has no grid active, so you can click the minus icon behind the grid section if it's active, like here: