Comment the adapter and env as well

symfony/messenger/4.1/config/packages/messenger.yaml

@@ -1,8 +1,9 @@
 framework:
     messenger:
+        adapters:
+            # Uncomment the following line to enable a default adapter
+            # default: "%env(MESSENGER_ADAPTER_DSN)%"
+
         routing:
             # Route your messages to the senders...
             # 'App\Message\YourMessage': messenger.default_sender
-
-        adapters:
-            default: "%env(MESSENGER_ADAPTER_DSN)%"

symfony/messenger/4.1/manifest.json

@@ -3,7 +3,7 @@
         "config/": "%CONFIG_DIR%/"
     },
     "env": {
-        "MESSENGER_ADAPTER_DSN": "amqp://guest:guest@localhost:5672/%2f/messages"
+        "#MESSENGER_ADAPTER_DSN": "amqp://guest:guest@localhost:5672/%2f/messages"
     },
     "aliases": ["messenger", "amqp"]
 }