SERV_MOD_ACT_TRACE_ENABLE problem
There is no need for the instance name, as you can see in the documentation


  This procedure will enable SQL tracing for a given combination of
  Service Name, MODULE and ACTION globally unless an instance_name is
  specified.


instance_name has a default value null implicating it is not a required parameter. In a RAC database this would just enable tracing for your service in all open instances.