Update: Pokémon UNITE maintenance has been completed as of December 8 at 11 p.m. PT, servers are back online and a new update with bug fixes is now live
Attention, Trainers! #PokemonUNITE will be in maintenance on December 8 from 1:00PM to 11:00PM PT. During this time, the game will not be playable while we make adjustments. We appreciate your patience and understanding!
Source: Official Pokémon UNITE website