A Unity ID allows you to buy and/or subscribe to Unity products and services, shop in the Asset Store and participate in the Unity community.
Separate names with a comma.
Weird. I just tested here: I created a new project, downloaded Standard Assets on it, closed it and when creating a new project it appears enabled...
I had the same problem but once you download it from the Asset Store, it is available in all projects to be imported into the "Add asset Package" tab.
Same for me, using Linux Mint 19, trying to install 2018.2.9f1