I am unable to install a .deb package downloaded from the internet.
In previous versions of Ubuntu, the software center would act as the GUI frontend to install deb packages but the new snap store does not support installation of deb package.
How do I install .deb package like Google Chrome or Zoom in Ubuntu 24.04 using GUI?
In a recent update, Ubuntu app center regained the functionality to handle deb files. But, if that does not work, try the rest of the answer.
First, install GDebi package manager with the command sudo apt install gdebi.
Afterward, you can open .deb packages with GDebi to install them.
Optionally, you can install GNOME Software (sudo apt install gnome-software) as a GUI appstore to browse/install/remove APT packages from the official repositories, and can also install .deb packages downloaded from the internet.