Can play training, but multiplayer won't start.

I just get the loading icon indefinitely. Waited 20 minsz started again and waited 10 more mins… nothing. Is this happening to anyone else?

Same issue here!

Yeah it was working a few hours ago, guess their matchmaking system must have gone down

Matchmaking is only available at certain times. It will open up again at 10 AM PT

The matchmaking only works during the playtest sessions this weekend/next weekend.

Outside of those hours, all you have is training mode to play round with

You can check times here
Content & Play Sessions – Halo Insider Support (halowaypoint.com)

"
This flight has scheduled play sessions that require you to queue during specific windows of time:

  • Friday, September 24th@ 10am-2pm and 5pm-9pm PT - Saturday, September 25th@ 10am-2pm and 5pm-9pm PT - Sunday, September 26th@ 10am-2pm and 5pm-9pm PT - Friday, October 1st@ 10am-2pm and 5pm-9pm PT - Saturday, October 2nd@ 10am-2pm and 5pm-9pm PT - Sunday, October 3rd@ 10am-2pm and 5pm-9pm PT"