Another problem is that nobody else (at this scale at least) got there so it's hard to anticipate all the challenges. Germany, normally at 36% renewable energy, managed to reach that magic 100% figure on a winter early-morning this year. But nothing close to sustained generation. And they plan to get to 100% in 2060+. So California, currently at ~44% renewable energy, could be the first to hit the sustained 100%.
Another problem is that everyone defines "100% clean energy" in different ways, by leaving some sectors out of the count. Will this target cover electrical power or also aims at replacing all non-renewable sources of power for all applications?