computeCartesianPath() returns -21 (FRAME_TRANSFORM_FAILURE)

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I'm trying to perform a linear motion to pick an object. This is code:

bool RoboticArm::linearMove(geometry_msgs::Pose pose) {
    move_group_ptr->setPlanningTime(10.0);

    robot_state::RobotState start_state(*move_group_ptr->getCurrentState());
    move_group_ptr->setStartState(start_state);

    std::vector<geometry_msgs::Pose> waypoints;

    geometry_msgs::Pose target_pose = pose;

    target_pose.position.z -= 0.15;

    waypoints.push_back(target_pose);  // down

    move_group_ptr->setPoseTarget(target_pose);

    move_group_ptr->setMaxVelocityScalingFactor(0.1);
    move_group_ptr->setPoseReferenceFrame("/camera_rgb_optical_frame");

    moveit_msgs::RobotTrajectory trajectory;
    const double jump_threshold = 0.0;
    const double eef_step = 0.01;
    moveit_msgs::MoveItErrorCodes error_code;
    double fraction = move_group_ptr->computeCartesianPath(waypoints, eef_step, jump_threshold, trajectory, true, &error_code);
    ROS_INFO_NAMED("tutorial", "Visualizing plan 4 (Cartesian path) (%.2f%% acheived) err(%d)", fraction * 100.0, error_code.val);


    if (move_group_ptr->execute(trajectory) != moveit::planning_interface::MoveItErrorCode::SUCCESS) {
        ROS_ERROR("Failed to plan linear motion");
        move_group_ptr->clearPoseTargets();
        move_group_ptr->setMaxVelocityScalingFactor(1.0);
        return false;
    }
    cout << "Linear Motion Completed" << endl;
    move_group_ptr->setMaxVelocityScalingFactor(1.0);
    return true;
}

We don't understand why the error is -21. Note that knowsFrameTransform() returns 1. Can someone help me?