Why can't my Arduino IDE recognize serial ports? (Mac El Cap, Ard. UNO)
"ATmega16u2 DFU" indicates a USB interface micro that is in a state to have new firmware downloaded to it (DFU), not one which is ready for operational use as a USB interface to program the Uno's ATmega328p target MCU.  If this persists after power-cycling/re-plugging the board, it is probably faulty and at minimum needs firmware repair - though it might be worth trying on another computer (preferably with a different OS or version) just to be absolutely sure nothing in the host software is forcing it to this mode.

If this is a new board you have not modified in any way, you should probably return it as defective.

If you have modified it, or you want to try to fix things yourself, you would need to follow the instructions for reflashing the 16u2 via USB-DFU.  Beware that this is a non-trivial task that looks like it is going to require installing macports and running some terminal commands, and on older boards some soldering.