To fix this issue your mod need a "multiplayer_object_types.bin" file included in your mod folder.

To get this is kind of a hassle but I found this tutorial page which explains how to get it.

https://learn.microsoft.com/en-us/halo-master-chief-collection/excession/appendixb

Once you have it you want to put it somewhere in your mod's folder.

Open the Reach Mod Uploader Tool and go to where you add your .map file. At the bottom there will be a button labeled "IMPORT MULTIPLAYER OBJECT TYPES". Click it and select your multiplayer_object_types.bin file.

Once this is done upload it to the workshop, download, and enjoy!