It means that there is no “sign up” option available when looking at your forum. Users need an invite link to sign up.
You can send that via email but also share the link elsewhere.
You can also invite users to this forum. You can find the link in the more menu of the sidebar, in your profile and the avatar menu.
The invite modal for admins offers a few more options, but the one here might work as an impression. Otherwise, just sign up for a free forum and start testing.