The online multiplayer servers for Splatoon on Wii U were shut down on March 3, 2023. This was due to a security issue that was discovered in the game’s code.
The shutdown affected all online features in Splatoon, including Turf War, Ranked Battle, and Splatfests. Players can still play the single-player campaign and local multiplayer modes.
You can also try playing Splatoon 2, which is the sequel to Splatoon and has all of the same features, plus more.