Error: expected primary-expression

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I am using the executors in rclcpp, but I keep on getting the following error:

 error: expected primary-expression before ‘exec’ rclcpp::executors::SingleThreadedExecutor exec;

This is exactly the same as in the example for the minimal_composition, so I 'm not sure why it is giving me the error. Does it have to do with these warning?

[0.247s] WARNING:colcon.colcon_ros.prefix_path.ament:The path '/home/overlord/orion_ros2/install/orion_sensor' in the environment variable AMENT_PREFIX_PATH doesn't exist
[0.247s] WARNING:colcon.colcon_ros.prefix_path.catkin:The path '/home/overlord/orion_ros2/install/orion_sensor' in the environment variable CMAKE_PREFIX_PATH doesn't exist