The Sandlot has been updated to support the latest Minecraft client, 1.12.2. You must now use this version to connect to and play on The Sandlot. The servers themselves may not be on that version (for example, there are no parrots on Easy) but that is the version you must use in your launcher. Let me know if you have any questions or run into any issues. Thanks.
I have now been trying for 30 minutes to log in with 1.12.1 without checking the website. I'm so smart. (Sarcasm) Anyway, thanks c0w!