Robin
it asks “please authenticate:”
It should ask you for your sudo password. Have you tried to enter it?
Alternatively you can do that easily on command line as well:
$ sudo apt-get update
$ sudo apt-get install '/path/to/downloaded/package.deb'1st step updates your local apt database copy, skip this if you have done that today already, needs merely to be repeated after a couple of hours usually, except you are awaiting a package updated most recently and the update is still not found. On live system after each reboot the database will be gone again, so here an update also is required. For installing from a locally provided package this is merely needed for the dependencies.
2nd step installs the locally provided .deb package. Path can be either relative, or absolute. But you must provide a path, even when the file resides in your current working directory (in this case use relative ./package.deb
Windows is like a submarine. Open a window and serious problems will start.