I tried to join a server, for example cubecraft. then when I pressed all of the servers, a message appeared "UNABLE TO CONNECT TO SERVER" it said, so HELP ME!!!!

Thank you for your report!
However, this issue is Invalid.

Unfortunately we don't accept issues related to External Servers here on the bug tracker, but please add more details if you think this also affects multiplayer over Realms, LAN, or XBL.

I'd suggest contacting the server owner directly or seeking help on the Minecraft forums, however in general you will not be able to play on featured servers with the beta version.

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