Due to the distribution policy of Oracle JDBC driver, it can only be downloaded manually from http://www.oracle.com/technetwork/database/features/jdbc/index-091264.html. After downloading, copy ojdbc6.jar file to the lib subdirectory of the CUBA Studio installation path and to the lib subdirectory of the installed Tomcat server. Then stop Studio, stop Gradle daemon by executing

in the command line, then start Studio again.