I see that you didn’t include the
* in the first attempt to run it. That symbol tells bash to run any AppImages that begin with Joplin without you having to type the full file name. It saves time and is just an advanced shortcut for what you’re trying to do. You don’t want to use it everywhere, just Electron builds only one AppImage.
Something I’m noticing is that you may not be all that experienced with running command line tools for development purposes. I would highly recommend before going any further with development here or elsewhere is to freshen up on how Linux terminals are ran and get a good idea of how to debug code and apps when running them from the terminal. The devs here are fairly patient but because the team is primarily one guy with multiple contributors, they may not have the time or resources to help with every issue that you are running into.