Run the database script <BPS<EI_HOME>/wso2/business-process/dbscripts/bps/bpmn/create/activiti.<DB_Name>.create.substitute.sql against your BPMN engine's database to add the Substitution related table.
Open the activiti.xml file found in the <BPS<EI_HOME>/wso2/repositorybusiness-process/conf/ directory and uncomment the following configuration.