Hi @bansi_lol,
Every ROS Distribution is targeted to a certain Ubuntu Distribution. ROS Indigo is targeted specifically for Ubuntu 14.04 LTS (Trusty). Since you are using Ubuntu 20.04 you will not be able to install apt binaries from Indigo, that is the reason of the "unable to locate package ros-indigo-desktop-full" error.
Since your enviroment is Ubuntu 20.04 you should use ROS Noetic. Besides that, Turtlebot project constains a lot of functionalities and tools that are stable in older ROS Distributions but that does not mean those are not available to use in newer ones.
All turtlebot packages are fully stable on ROS Melodic so if you want to have those in a Noetic Distribution you may want to clone all the repositories in you catkin workspace and compile them in you enviroment. If you install all needed dependencies you should not have any problem since, generally speaking, any package stable in Melodic is likely to work properly under Noetic.
