Creating a Flexbox from the 'Flexbox' Element
Click on the Elements icon, which lies on your right sidebar.
Flexbox will be the fourth one on the list, as you can see, it's named!
Drag the flexbox element and drop it to your canvas.
All done!
How to Transform Any Existing Block to a Flexbox
Click on Elements, on your ride sidebar.
Drag any block and drop it on your canvas
Go to the Layout Settings, on your right side
Scroll down to find a field named Display
Click on the dropdown button, that lies right to the Display.
Choose Flex from the list
Tada! Now your Div is a Flex!