/errors/ad_error.proto 

--- v20/errors/ad_error.proto   2025-08-05 14:39:41.000000000 +0000
+++ v21/errors/ad_error.proto   2025-08-05 14:39:47.000000000 +0000
@@ -522,5 +522,9 @@

     // Images must be unique between different carousel card assets.
     DUPLICATE_IMAGE_ACROSS_CAROUSEL_CARDS = 156;
+
+    // For video ads sequencing, YouTube video asset ID has to be defined in
+    // `campaign.video_campaign_settings.video_ad_sequence.steps.asset_id`.
+    INVALID_YOUTUBE_VIDEO_ASSET_ID_FOR_VIDEO_ADS_SEQUENCING = 157;
   }
 }