/errors/experiment_error.proto 

--- v15/errors/experiment_error.proto   2024-02-20 20:29:00.000000000 +0000
+++ v16/errors/experiment_error.proto   2024-02-20 20:29:02.000000000 +0000
@@ -124,5 +124,10 @@
     // Cannot add campaign with deprecated ad types. Deprecated ad types:
     // ENHANCED_DISPLAY, GALLERY, GMAIL, KEYWORDLESS, TEXT.
     CANNOT_ADD_CAMPAIGN_WITH_DEPRECATED_AD_TYPES = 28;
+
+    // Sync can only be enabled for supported experiment types. Supported
+    // experiment types: SEARCH_CUSTOM, DISPLAY_CUSTOM,
+    // DISPLAY_AUTOMATED_BIDDING_STRATEGY, SEARCH_AUTOMATED_BIDDING_STRATEGY.
+    CANNOT_ENABLE_SYNC_FOR_UNSUPPORTED_EXPERIMENT_TYPE = 29;
   }
 }