KjayCopper hi,
if you want to install the plugin for your main website without composer you can try via this website, that downloads it for you (however, it requires a subscription to download ipackages): https://php-download.com/package/maicol07/flarum-sso-plugin
You can also try to download the repo zip and also its dependencies, but then you would check all the use statement and edit them with your path, which may not be worth the time. If you still want to try, here's the repo: https://github.com/maicol07/flarum-sso-php-plugin
I recommend you to learn how to use Composer, since it's very easy and simplifies a lot packages management
