Seems like I got errors very much like that when I installed on Mint 19.3 Cinnamon. However, I am still able to start and run it by navigating in terminal to the nanovna-saver folder and entering "python3.7 nanovna-saver.py". I think with earlier versions I had to use "python3.7 ./nanovna-saver.py" but I could be remembering wrong. I am using nanovna-saver 0.2.2.
Jim Allyn - N7JA