Postman not launching anymore in 18.04
Solution 1:
I had the same problem on a clean 18.04 install, the library libgconf2-4 could not be found.
Try sudo apt install libgconf2-4, it should solve your problem.
Solution 2:
Slight change to the answer provided by @Stanko worked for me.
Try adding hypen(-) in between libgconf and 2
sudo apt install libgconf-2-4