There are many questions about Unable to locate package.

For example : if user try to install package by using apt-get then the following error shown. (Here package is used only as example for displaying error message):

sudo apt-get install package
Reading package lists... Done
Building dependency tree       
Reading state information... Done
E: Unable to locate package package

What does this error mean? How do I solve it?

Here provided General solution to understand and how to solve such type of error (which is helpful for new users that may unaware of repository-apt install mechanism)

Note : For resolving unmet-dependencies, visit: This question.