From Nintendo,  all online service for ARMS will temporarily be down for maintenance.

The downtime will take place on the following day and time:

Europe: July 12th, 0250hrs to 0430hrs
UK: July 12th, 0150hrs to 0330hrs
Eastern Standard Time (EST): July 11th, 2050hrs to 2230hrs
Pacific Standard Time (PST): July 11th, 1750hrs to 1930hrs
Japan: July 12th, 0950hrs to 1130hrs

Singapore: July 12th, 0850hrs to 1030hrs

Look for other entertainment like Breath of the Wild DLC Pack 1 to bide your time while the maintenance undergoes.