- The server is running a different version to the client. Ensure the Minecraft Client is running the exact same version as the server.
- The IP is being incorrectly input. Always ensure you add the port on the end, if you do not have 25565.
- If the server is a modded/forge server, ensure the client as the same version of everything installed.
- Check that any firewalls or anti-viruses aren't blocking the connection to the server. Test connecting to our Test Server.
If after trying all these things, you're still having issues, please send in a Support Ticket.