Congratulations on learning the basics of CSS and Flexbox. You're one step closer to be able to build websites on your own. Because Flexbox is so important, we have a fun little game to play that will really improve your understanding of how Flexbox works. Head over to www.flexboxfroggy.com and test your flexbox skills.
Towards the end gets really difficult, and you are absolutely not expected to understand how to solve it - 99.9% of experienced frontend developers can't do the whole thing. Feel free to check out the solutions here if you get stuck. Good luck.