Whereabout are you located ?
We had that error code in 2020; players in South America couldn’t launch D3 using their usual server (in Brazil, most probably) but could if they forced the use of the server in the US by adding the “OnlineService.Matchmaking.ServerPool=Default” argument.
See this post for a possible workaround: Error Code 30002 - #7 by Paton812-1159
Good luck !