Installing applications from the downloaded .deb files in Ubuntu has been a pain lately,
Earlier, just double clicking it on a .deb file would open it for installation in the Software Center.
Later, it was changed a bit and you would have to right-click open with Software Center.
Now the Snap based Software Center refuses to install the third-party .deb files.
The alternate method is to install deb file using a GUI tool like Gdebi or use the commands like apt or dpkg.