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As @tfoote pointed out, you have to add geometry_msgs package to the dependencies.

However, even after that, there are 2 errors in your C++ code

1. RCLCPP_INFO("derp"); This is not allowed, as the first argument to RCLCPP_* should be a logger object.

2. msg->linear[0] this is not correct. The linear field of the message is of type Vector3, but it's not a vector. Look at the implementation of this type here and you will see that you have to access to its fields using msg->linear.x, msg->linear.y, msg->linear.z