Elementary OS6 Stable Fresh install cannot install deb file and no Eddy on AppCenter, please help?
If you decide to stick to this latest eOS crap you can do this:
sudo apt install gdebiand it won't work out-of-the-box.
sudo nano /usr/share/applications/gdebi.desktop
Exec=sh -c "gdebi-gtk %f"
Gdebi is far better than AppCenter or Eddy because it will tell you what dependencies are needed for the installation (like the all KDE environment just to get a simple app) and, more importantly, are there gonna be any packages removed as a result of installation which is something that eOS users have experienced.