Hi there,
If you could, can you go to Tools->Port and see if there is any ports listed. The error message that you posted indicates that the port that is selected is not what your Uno is connected to. If the board is a genuine Uno, in the Tool->Port menu it will list something along the lines of Com 7 (Arduino/Genuino Uno). It may not say 7, that’s just a random number.
Give it a try and let me know!