Unable to install any package from apt (Package not available)

For the past week, I have been facing this issue, for many packages that I try to install from apt.

Reading package lists... Done
Building dependency tree       
Reading state information... Done
Package **package_name** is not available, but is referred to by another package.
This may mean that the package is missing, has been obsoleted, or
is only available from another source

This issue is with every package.

Just to mention. I have tried all the solution that has been mentioned here "But is referred to by another package." - Finding that package.

But nothing is working. I am really stuck with this package thing. My system.

Description:    Ubuntu 20.04.3 LTS
Release:    20.04
Codename:   focal

Reading package lists... Done
Building dependency tree       
Reading state information... Done
Package libwxgtk3.0-dev is not available, but is referred to by another package.
This may mean that the package is missing, has been obsoleted, or
is only available from another source

E: Package 'libwxgtk3.0-dev' has no installation candidate

Solution 1:

The package libwxgtk3.0-dev is not available for Ubuntu 20.04.

I guess the documentation for your project is written for Ubuntu 18.04, for which that package name exists.

However, you can use the replacement package libwxgtk3.0-gtk3-dev.
Note: It is universe repository, which you might need to enable:

sudo apt-add-repository universe

For such issues, https://packages.ubuntu.com/ is a good website to start package research.