Since you mention that you are using Ubuntu 20.04
, you can not be followinguse the tuorialtutorial. 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 nonot supported any more (https://docs.ros.org/en/rolling/Releases.html)