When I want to install an other/older version of Minecraft under the installations tab -> create new installation, only versions I have previously installed are visible. I can not install other versions sense they do not show up as an option.

I have previously been able to install other versions of Minecraft, such as 1.8.9. Other versions have showed up as options under the installations tab -> create new installation, but, they do not show up anymore. 

I do not know what happened/ what changed to make it so that I can't create new installations, such as 1.15.1, that are not once I already installed.