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  There are few logs more disheartening in mobile development than   Error Code 1001: No Fill . You have integrated the SDK, followed the documentation, and successfully initialized the library. Yet, when you request an ad, the   onError   callback fires immediately. For developers aiming to monetize via the Meta (Facebook) Audience Network, this error is the primary blocker between development and revenue. The good news is that in 90% of cases, your integration code is perfectly fine. The issue lies in the configuration or the inventory supply chain. This guide provides a rigorous technical breakdown of why  1001  occurs and details the specific, code-level solutions to resolve it for Android and iOS in 2025. The Root Cause: What is Error 1001? To fix the error, you must understand the mechanism behind it. Error 1001 does not mean the SDK failed to communicate with Meta's servers. It means the server received your request but chose not to return an ad. The...