Question Question: How to add build modules?

Just to start with a little backstory: I am a high school student currently without any source of income as I am moving and own two laptops. The first, which I am using now is an old one which used to run Windows Vista and because it is so old can only run 32bit programs. The second one is a refurbished one I got off of Ebay for around 150 dollars and can run 64bit programs pretty well. The second one randomly stopped working and won't even charge let alone turn on, so I have resorted to using this one, which crashes a lot. Because of this the latest version of Unity I can have installed is version 5.6.7f1.

I am currently making my first ever game, (a visual novel) and using my friend as a beta tester. He doesn't have a conventional computer and uses a Chromebook he got for 20 dollars from the school when he graduated. I was planning on exporting the game as a WebGL so he could play but found out you can only dt=o that with 64bit unity. I only have the Android, IOS, WebGL, and Windows build module installed and can't figure out how to install the Linux build module so he can download and play the game.

I have tried logging out and back in and uninstalling and reinstalling the unity version. Neither has worked. The following picture is a screenshot of what I see. Can someone please help me figure this out so my friend can play my game?