Class not found exception displayed when required driver class jar file is not added in opnbi. by default, opnbi setup comes with PostgreSQL and MySQL database connection jars. if you are trying to connect with some other database like SQL Server or MongoDB etc. you have to download the database connection jar files and place them in to <your directory>/opnbi/tomcat/webapps/opnbi/WEB-INF/lib/ folder. after adding jar file, restart tomcat server once for changes to reflect. check test connection again after server restart.
This might help you to solve your error.