[ROS Warning] Unable to locate package velodyne_test build file

edit

Yes, but I remember that before reinstalling it and running roscd the result was I believe /home/emanuele/catkin_ws/devel. Is it because I sourced it differently? How can I make the compiler find it then? Do I have to reinstall ROS again?

You have to source that for every terminal you use in every session

As of now what I do is: emanuele@pc:~/catkin_ws$ catkin_make and it works and compile correctly on terminal. But when I try to build in Qt5 it gives me the messages I posted on the question. I am not sure of what I should do to have Qt5 compiling and building it.

I mean if I try to do this emanuele@pc:/opt/ros/melodic$ catkin_make the terminal says:

Base path: /opt/ros/melodic

The specified source space "/opt/ros/melodic/src" does not exist

try opening .bashrc by gedit ~/.bashrc then add the below line at the bottom of the file opened source "/home/emanuele/catkin_ws/devel/setup.bash" then compile from catkin_ws

@Abhishekpg111, thank you for your comment. It partially worked! Now it compiles via terminal but I still Qt is not compiling. The problem that is left is that Qt it seems can't find the executable. Where can I find it? Is this and this useful to better understand?

Could the executable be inside the devel folder? Here is a print-screen if that could be helpful

is your velodyne_simulator package under catkin_ws/src folder ??