I followed the instructions here.It asked me to enter the following commands:
chmod +x ./QGroundControl.AppImage
I got this error:
QGC did start but I couldn't connect to my Pixhawk. So I took it's advice and entered the commands:
sudo usermod -a -G dialout $USER
sudo apt-get remove modemmanager
I closed QGC and tried to restart it but it no longer works. Nothing happens every time I go to the download directory and execute the AppImage by typing in ./QGroundControl.AppImage. I also tried to right click the app image and click on run. Nothing happens.
P.S. I am using Ubuntu 18.04 LTS
UPDATE: Don't know why or how but disconnecting everything and rebooting made it work again. Neither Ubuntu, nor the GQC installation guide ever mentions that a reboot is required. I hope this post will help anyone else who is going through similar problems.