The robot pose ekf will not start when it only receives messages on this imu's topic, it also expects messages on either the 'vo' or the 'odom' topic. So you need a odometry topic in order to get it working
Subscribing to odometry topic publisher, adding covariance to the odometry message then republishing the message may solve your problem in your case. Or just edit the source code of the publisher and add some covariance
covariance is added by like this ( it is just an example )
nav_msgs::Odometry odom;
odomQuaternion = tf::createQuaternionMsgFromYaw(poseAngle);
odom.header.stamp = _currentTime;
odom.header.frame_id = "odom";
odom.pose.pose.position.x = poseX;
odom.pose.pose.position.y = poseY;
odom.pose.pose.position.z = 0.0;
odom.pose.pose.orientation = odomQuaternion;
odom.pose.covariance[0] = 0.01;
odom.pose.covariance[7] = 0.01;
odom.pose.covariance[14] = 99999;
odom.pose.covariance[21] = 99999;
odom.pose.covariance[28] = 99999;
odom.pose.covariance[35] = 0.01;
odom.child_frame_id = "base_footprint";
odom.twist.twist.linear.x = vx;
odom.twist.twist.linear.y = 0;
odom.twist.twist.angular.z = w;
odom.twist.covariance = odom.pose.covariance;
