I have found my mistake. For example, if I want to install "Develop", a simple "sudo apt install develop" is not enough, I have to give the full name, which I find out with "sudo apt search " for example.
In my example "sudo apt install com.github.alcadica.develop".
@George Yes, of course, this works wonderfully for programs from the Ubuntu sources. But not for those from the elementary AppCenter (maintained applications like "Develop"). Here the package is not found and you can (must) install it with the graphical tool.