I am trying to install ROS on ubuntu 21.04 but this error is shown to me when I run sudo apt update on the terminal. Also, I tried to use the Mirror repo but the same error is shown to me.
E: The repository 'http://packages.ros.org/ros/ubuntu hirsute Release' does not have a Release file.
N: Updating from such a repository can't be done securely, and is therefore disabled by default.
N: See apt-secure(8) manpage for repository creation and user configuration details.
There is no ROS release for 21.04. The latest distro is Noetic which targets 20.04. You'll either need to downgrade or try to install it in some 20.04 container(i.e. docker)