Since you mention that you are using `Ubuntu 20.04`, you can not be following the tuorial. It is written for `Ubuntu 22.04`  (https://micro.ros.org/docs/tutorials/core/first_application_rtos/freertos/#:~:text=Hawksbill%20on%20your-,Ubuntu%2022.04,-LTS%20computer.%20To)

I assume you are trying to use **galactic**, which is no supported (https://docs.ros.org/en/rolling/Releases.html)