The library doesn't currently support the Uno WiFi Rev2. The library authors don't seem very motivated to add support. The best bet is for some community member to do the necessary work and then submit a pull request to the FastLED library. Hopefully that will happen soon.
Although the Arduino system is designed to offer the highest level of compatibility between different architectures, in some cases you will find that libraries or sketches contain architecture-specific code that must be updated to work with a different board. For this reason, the community support for new Arduino boards takes a little while to catch up. You will find the best support for the AVR boards simply because they've been around the longest. For someone who enjoys challenges and wants to make significant contributions to the Arduino project, the new hardware is a great choice. For a beginner who just wants things to work immediately with the minimum of complexity, it may not be.