If a community appears as a 404 page when you click it, it likely means that the community is being hosted outside of the instance you are currently using and that nobody in your instance has viewed or interacted with that specific community yet. In order to address this, you will need to copy-paste the specific handle for the community (for example, [email protected]) into your searchbar so that your instance can “learn” that it exists, after which the community should display via linked URLs without issue. Hopefully this is addressed by the lemmy devs and streamlined in the near future.