Caused by: org.apache.commons.dbcp.SQLNestedException: Cannot create PoolableConnectionFactory (SELECT command denied to user 'sonar_user'@'localhost' for table 'session_variables') at org.apache.commons.dbcp.BasicDataSource.createPoolableConnectionFactory(BasicDataSource.java:1549) ~[commons-dbcp-1.4.jar:1.4] at org.apache.commons.dbcp.BasicDataSource.createDataSource(BasicDataSource.java:1388) ~[commons-dbcp-1.4.jar:1.4] at org.apache.commons.dbcp.BasicDataSource.getConnection(BasicDataSource.java:1044) ~[commons-dbcp-1.4.jar:1.4] at org.sonar.core.persistence.DefaultDatabase.checkConnection(DefaultDatabase.java:113) ~[sonar-core-4.5.5.jar:na] ... 27 common frames omitted Caused by: com.mysql.jdbc.exceptions.jdbc4.MySQLSyntaxErrorException: SELECT command denied to user 'sonar_user'@'localhost' for table 'session_variables' at sun.reflect.NativeConstructorAccessorImpl.newInstance0(Native Method) ~[na:1.8.0_60] at sun.reflect.NativeConstructorAccessorImpl.newInstance(NativeConstructorAccessorImpl.java:62) ~[na:1.8.0_60] at sun.reflect.DelegatingConstructorAccessorImpl.newInstance(DelegatingConstructorAccessorImpl.java:45) ~[na:1.8.0_60] at java.lang.reflect.Constructor.newInstance(Constructor.java:422) ~[na:1.8.0_60] at com.mysql.jdbc.Util.handleNewInstance(Util.java:411) ~[mysql-connector-java-5.1.27.jar:na] at com.mysql.jdbc.Util.getInstance(Util.java:386) ~[mysql-connector-java-5.1.27.jar:na] at com.mysql.jdbc.SQLError.createSQLException(SQLError.java:1054) ~[mysql-connector-java-5.1.27.jar:na] at com.mysql.jdbc.MysqlIO.checkErrorPacket(MysqlIO.java:4237) ~[mysql-connector-java-5.1.27.jar:na] at com.mysql.jdbc.MysqlIO.checkErrorPacket(MysqlIO.java:4169) ~[mysql-connector-java-5.1.27.jar:na] at com.mysql.jdbc.MysqlIO.sendCommand(MysqlIO.java:2617) ~[mysql-connector-java-5.1.27.jar:na]
Your SonarQube server connects to the database with a user ('sonar_user'@'localhost') who does not have sufficient privileges (as per error message, it cannot issue SELECT commands in the database).
You need to grant this permission to this user in the MySQL DB (here's a helpful tutorial on how to achieve that).