This error means your Xbox Live account is restricted from accessing user-generated content (Forge maps, custom games, file shares). Here are the most common causes and fixes.
Halo: Reach on Xbox 360 runs on deprecated services. Some UGC features (file share browsing, downloads) may be partially broken even with correct settings. The supported way to play Forge today is Halo: The Master Chief Collection on Xbox One/Series/PC.
If settings are correct and the error persists, contact Xbox Support.