Hey! So, lemme explain:
You can only see communities and easily subscribe if at least 1 person in your instance has done it already.
If you're the first one from your instance to do so, what you need to do is copy the full URL (https://sh.itjust.works/c/shadowrun, for example), go to the communities list at the top of the site, hit search to get the options up, change from "Communities" to "All", then wait for your instance to pull in the content (maybe 5-10 seconds or so)
Finally, click through to the Community and click subscribe. Now that community is pulled onto your server for yourself and everyone else in your server.
Can provide images and additional clarification if needed. I know it's a little janky and unintuitive, but a Lemmy server can't just pull in all communities from all servers all the time, they'd explode. There needs to be at least 1 person who wants to see that community (subscribed) for the server to know to do that.