You're probably playing on 1.13...the current version of Minecraft. The Sandlot isn't updated yet (and won't be until all of the plug-ins are updated as well). You'll have to switch to a 1.12.2 version of Minecraft to play on the Sandlot.
If anyone else finds this post and has the same problem, please see here for help: https://www.sandlotminecraft.com/threads/outdated-server-how-to-play-on-the-sandlot.33284/