There is a glitch in the Reclaimer level where the board disappears, and you can go any further in the level.
I have the same problem and I can’t find how to fix it, I’ve tried everything
Found this on the Halo Support site:
2020 November 17
In the first third of the Halo 4 campaign mission “Reclaimer,” around the start of Rally Point Bravo, the Covenant’s Lich appears overhead. Sometimes after completing this encounter, the Lich will fail to explode and the remainder of the mission will unload. This unloaded area appears as a large void and blocks all progress in the mission.
To progress past this point in the mission, you can try restarting the mission from the beginning or restarting from Rally Point Bravo. Restarting from the previous checkpoint of your current progress will not resolve the issue.