Hi, welcome to the community!
AppVM gets its root filesystem from another qube (template qube), Fedora (or Debian), by default. So whenever you want to introduce changes into the root filesystem (including installing something) you have to do it in the latter qube. After that you will have to shutdown the TemplateVM and reboot the AppVM. This is how Qubes’ isolation works.
More details:
But be careful: Installing anything unsafe in TemplateVM will make all AppVMs relied on it unsafe. More details:
Unless you are willing to learn a lot, it might be easier for you to switch to some GNU/Linux distribution first (e.g., Fedora or Debian), before dealing with Qubes OS. The latter introduces more complexity and relies on some knowledge of how Linux works.