Flexbox Froggy
Flexbox Froggy is a fun simple game where you use CSS flex properties to move frogs to their lily pads. The lily pads change location each level and the user has to select the correct flex property to move each frog to the correct lily pad.
Coding Fantasy - Flexbox Adventure
Flexbox Adventure is a game where King Arthur is chasing after some alchemist brothers to reclaim his gold with his powerful weapon - CSS Flexbox. In the game, you use flex properties to help King Arthur on his journey with various tasks such as picking up items and more.
Codeacademy
Codeacademy has several courses and demos that you can access for free upon creating an account. There are other features that can be accessed if you have a paid plan, but you get a lot of them on the free plan. The flex course has a lot of detail and the lessons are very interactive where you can edit and run the code.
Summary
Overall, there are lots of resources online that can be accessed for free to learn flex. Many of them are games which can help users learn coding better. Out of the three resources above, I recommend using Flexbox Froggy to learn flex because of how simple, fun, and straightforward it is.