As the game is in early access, it's very likely there will be frequent updates, if the server files do not match client files the server will not show up in the list, log in to your control panel and press the steam update button.
Invalid server name:
There are several characters that can cause the server to now show up on the list such as # ~ @ . If your server is not showing remove any characters, keep it to just letters and reboot the server to test.
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