Account data will not sync after installation

I have not played Halo 5 in over a year. I just downloaded it, and it will not allow me to play because it won’t sync my account data.

Hey OP, looks like there is a known issue right now. You can follow this thread for updates
https://www.halowaypoint.com/en-us/forums/e0d9938206f04a598ce84f2c8cadce02/topics/unable-to-access-online-game-modes-and-features/a71aadd3-ec5c-4a46-b5b1-0b01208084bd/posts?page=1#post2