Fix string formatting error

Change-Id: I8ef4dc2974f817d2bf13a8da612c0190e96ad098
diff --git a/TestON/drivers/common/api/controller/trexclientdriver.py b/TestON/drivers/common/api/controller/trexclientdriver.py
index 1150d4c..f6dfffe 100644
--- a/TestON/drivers/common/api/controller/trexclientdriver.py
+++ b/TestON/drivers/common/api/controller/trexclientdriver.py
@@ -322,8 +322,7 @@
             trex_daemon_client.start_stateless(cfg=trex_config_file_on_server)
         except ConnectionRefusedError:
             main.log.error(
-                "Unable to connect to server %s.\n" +
-                "Did you start the Trex daemon?" % trex_address)
+                "Unable to connect to server %s.\nDid you start the Trex daemon?" % trex_address)
             return False
 
         return True