/errors/recommendation_error.proto 

--- v19/errors/recommendation_error.proto   2025-08-05 14:37:33.000000000 +0000
+++ v20/errors/recommendation_error.proto   2025-08-05 14:37:45.000000000 +0000
@@ -164,5 +164,9 @@
     // recommendation type for bidding strategy type TARGET_IMPRESSION_SHARE.
     CAMPAIGN_BUDGET_RECOMMENDATION_TYPE_WITH_CHANNEL_TYPE_SEARCH_AND_BIDDING_STRATEGY_TYPE_TARGET_IMPRESSION_SHARE_REQUIRES_TARGET_IMPRESSION_SHARE_INFO =
         35;
+
+    // Merchant Center Account ID is only supported for advertising_channel_type
+    // PERFORMANCE_MAX.
+    MERCHANT_CENTER_ACCOUNT_ID_NOT_SUPPORTED_ADVERTISING_CHANNEL_TYPE = 36;
   }
 }