NOOBS allocates all of the available SD card space to the initial operating systems you install. So there is no space left to install more operating systems afterwards. To do that you have to delete what is there and start again.
You could try PINN which is a fork of NOOBS that includes many more useful features. It allows you to reserve some spare space on the SD card called "ProjectSpaces" that you can later replace with different operating systems. You can also use it to backup and restore installed operating systems to USB stick, which is another way to help add more operating systems to your SD card without losing your existing operating systems (although you stil need to take them off and put them back again).
PINN installs the same operating systems as NOOBS plus many others.
You can find detailed instructions here:
https://github.com/procount/pinn/blob/m ... ME_PINN.md
A quickstart guide is here:
You can download the lite, full or Etcher images from here:
https://sourceforge.net/projects/pinn/f ... rce=navbar