Just as the title says 
Could you please elaborate?
Cant play any multiplayer on reach, but can on any other game.
You still haven’t elaborated, no error message? are you using the account that has xbox live gold on it?
Can you play multiplayer on other backwards compatible games?
Have you been able to play multiplayer through backwards compatibility in the past?
How is anyone supposed to help if you don’t tell them anything.
You post is just pointless complaining until you give details, chances are you are doing something wrong and it will be an easy fix
Also if you are looking for support, use the appropriate forum