install ros-melodic-desktop-full error

edit

I did 1.2 Setup you sources.list, then I did 1.3 Set up your keys, then I did Installation. All seemed like it was going well. Then i entered the command

sudo apt-get install ros-melodic-desktop-full.

I don't get far:

Reading package lists... Done
Building dependency tree
Reading state information... Done
E: Unable to locate package ros-melodic-desktop-full.

Now I'm sad. I've had ros kenetic on my system working well. But I don't know what to do. Remove ROS again? And try adding pixie dust or water from the river jordan? If I can not fix this I may go back to loading ROS kenetic, which was a successful install.