is it possible (and how?) to install AppCenter Apps (like Quilter or Agenda) via command line? sudo apt install doesn't work for me...
sudo apt install name-of-package is the right way to do it.
One way to go about this is to double-check that you are giving the right package name.
You can look for the desired package/app with:
apt search name-of-package
If that doesn't work for you then reply with the error that you are getting in terminal.